If you check the clock, does the time update? If it does, then it is not "freezed" but it is just waiting some input (some hidden modal dialog may be).
The backtrace doesn't seem like a freeze; what command did you used to fetch it on GDB "bt" or "bt full" ? (you should use the last one).
Thanks

Sorry, yes it does not freeze.. it just waits for input.

full backtrace:

#0  0xb7fc8424 in __kernel_vsyscall ()
No symbol table info available.
#1  0xb612adc6 in poll () from /lib/libc.so.6
No symbol table info available.
#2  0xb4d6b2ab in g_poll () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#3  0xb4d5e21b in g_main_context_iterate () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#4  0xb4d5e513 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#5  0xb64634ec in QEventDispatcherGlib::processEvents () from /usr/lib/libQtCore.so.4
No symbol table info available.
#6  0xb67590b5 in ?? () from /usr/lib/libQtGui.so.4
No symbol table info available.
#7  0xb64370f9 in QEventLoop::processEvents () from /usr/lib/libQtCore.so.4
No symbol table info available.
#8  0xb6437542 in QEventLoop::exec () from /usr/lib/libQtCore.so.4
No symbol table info available.
#9  0xb7ab0300 in KIO::NetAccess::enter_loop () from /usr/lib/libkio.so.5
No symbol table info available.
#10 0xb7ab0b3e in KIO::NetAccess::synchronousRunInternal () from /usr/lib/libkio.so.5
No symbol table info available.
#11 0xb7ab12cb in KIO::NetAccess::synchronousRun () from /usr/lib/libkio.so.5
No symbol table info available.
#12 0xb7c7427d in KFileWidget::KFileWidget () from /usr/lib/libkfile.so.4
No symbol table info available.
#13 0xa863e40b in KFileModule::createFileWidget () from /usr/lib/kde4/libkfilemodule.so
No symbol table info available.
#14 0xb7b2a815 in KFileDialog::KFileDialog () from /usr/lib/libkio.so.5
No symbol table info available.
#15 0xa86db0b5 in Image::showFileDialog () from /usr/lib/kde4/plasma_wallpaper_image.so
No symbol table info available.
#16 0xa86dc983 in Image::qt_metacall () from /usr/lib/kde4/plasma_wallpaper_image.so
No symbol table info available.
#17 0xb644e81c in QMetaObject::activate () from /usr/lib/libQtCore.so.4
No symbol table info available.
#18 0xb644ec68 in QMetaObject::activate () from /usr/lib/libQtCore.so.4
No symbol table info available.
#19 0xb6d6a5f1 in QAbstractButton::clicked () from /usr/lib/libQtGui.so.4
No symbol table info available.
#20 0xb6a96f39 in ?? () from /usr/lib/libQtGui.so.4
No symbol table info available.
#21 0xb6a98b94 in ?? () from /usr/lib/libQtGui.so.4
No symbol table info available.
#22 0xb6a98e21 in QAbstractButton::mouseReleaseEvent () from /usr/lib/libQtGui.so.4
No symbol table info available.
#23 0xb6b7e33c in QToolButton::mouseReleaseEvent () from /usr/lib/libQtGui.so.4
No symbol table info available.
#24 0xb670fe52 in QWidget::event () from /usr/lib/libQtGui.so.4
No symbol table info available.
#25 0xb6a96dde in QAbstractButton::event () from /usr/lib/libQtGui.so.4
No symbol table info available.
#26 0xb6b80cea in QToolButton::event () from /usr/lib/libQtGui.so.4
No symbol table info available.
#27 0xb66bb554 in QApplicationPrivate::notify_helper () from /usr/lib/libQtGui.so.4
No symbol table info available.
#28 0xb66c3590 in QApplication::notify () from /usr/lib/libQtGui.so.4
No symbol table info available.
#29 0xb75b781a in KApplication::notify () from /usr/lib/libkdeui.so.5
No symbol table info available.
#30 0xb6438b4b in QCoreApplication::notifyInternal () from /usr/lib/libQtCore.so.4
No symbol table info available.
#31 0xb66c2553 in QApplicationPrivate::sendMouseEvent () from /usr/lib/libQtGui.so.4
No symbol table info available.
#32 0xb672ff34 in ?? () from /usr/lib/libQtGui.so.4
No symbol table info available.
#33 0xb672f415 in QApplication::x11ProcessEvent () from /usr/lib/libQtGui.so.4
No symbol table info available.
#34 0xb6759982 in ?? () from /usr/lib/libQtGui.so.4
No symbol table info available.
#35 0xb4d5ad98 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#36 0xb4d5e3e0 in g_main_context_iterate () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#37 0xb4d5e513 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#38 0xb64634ec in QEventDispatcherGlib::processEvents () from /usr/lib/libQtCore.so.4
No symbol table info available.
#39 0xb67590b5 in ?? () from /usr/lib/libQtGui.so.4
No symbol table info available.
#40 0xb64370f9 in QEventLoop::processEvents () from /usr/lib/libQtCore.so.4
No symbol table info available.
#41 0xb6437542 in QEventLoop::exec () from /usr/lib/libQtCore.so.4
No symbol table info available.
#42 0xb643989f in QCoreApplication::exec () from /usr/lib/libQtCore.so.4
No symbol table info available.
#43 0xb66bb3d7 in QApplication::exec () from /usr/lib/libQtGui.so.4
No symbol table info available.
#44 0xb7f57b76 in kdemain () from /usr/lib/libkdeinit4_plasma.so
No symbol table info available.
#45 0x0804871b in _start ()
No symbol table info available.

Versions:

Qt 4.5.1
Glibc 2.10.1

df:

/dev/sda3             40322028   7609924  30663820  20% /
none                   1686880         0   1686880   0% /dev/shm
/dev/sda4            113045092   1877748 105424912   2% /home
/dev/sda1               202225     16076    175707   9% /boot
/dev/sdc1            488384000 207466368 280917632  43% /media/storage

Added Symbols... No qt debug symbols as archlinux qt does not have a package for qt debug symbols.

#0  0xb7fcd424 in __kernel_vsyscall ()
No symbol table info available.
#1  0xb612fdc6 in poll () from /lib/libc.so.6
No symbol table info available.
#2  0xb4d752ab in g_poll () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#3  0xb4d6821b in g_main_context_iterate () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#4  0xb4d68513 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#5  0xb64684ec in QEventDispatcherGlib::processEvents () from /usr/lib/libQtCore.so.4
No symbol table info available.
#6  0xb675e0b5 in ?? () from /usr/lib/libQtGui.so.4
No symbol table info available.
#7  0xb643c0f9 in QEventLoop::processEvents () from /usr/lib/libQtCore.so.4
No symbol table info available.
#8  0xb643c542 in QEventLoop::exec () from /usr/lib/libQtCore.so.4
No symbol table info available.
#9  0xb7ab5300 in KIO::NetAccess::enter_loop (this=0xbfee0e64)
    at /home/jan/kdemod/core/kdelibs/src/kdelibs-4.2.4/kio/kio/netaccess.cpp:502
        eventLoop = <incomplete type>
#10 0xb7ab5b3e in KIO::NetAccess::synchronousRunInternal (this=0xbfee0e64, job=0xa54e4a8, window=0x0, data=0x0,
    finalURL=0x0, metaData=0x0) at /home/jan/kdemod/core/kdelibs/src/kdelibs-4.2.4/kio/kio/netaccess.cpp:487
        meta = (const QMetaObject *) 0xb7c1a040
        dataSignal = "data(KIO::Job*,QByteArray)"
        redirSignal = "redirection(KIO::Job*,KUrl)"
#11 0xb7ab62cb in KIO::NetAccess::synchronousRun (job=0xa54e4a8, window=0x0, data=0x0, finalURL=0x0, metaData=0x0)
    at /home/jan/kdemod/core/kdelibs/src/kdelibs-4.2.4/kio/kio/netaccess.cpp:277
        kioNet = {<> = {<No data fields>}, static staticMetaObject = {d = {superdata = 0xb6528d08,
      stringdata = 0xb7bc6bc0 "KIO::NetAccess", data = 0xb7bc6ca0, extradata = 0x0}}, d = 0xa54d410}
        wasAutoDelete = <value optimized out>
        ok = 252
#12 0xb7c7927d in KFileWidget (this=0x9ef9008, _startDir=@0xbfee124c, parent=0xa1b8040)
    at /home/jan/kdemod/core/kdelibs/src/kdelibs-4.2.4/kfile/kfilewidget.cpp:581
        res = false
        startDir = {<QUrl> = {d = 0xa554358}, d = 0x0}
        filename = {static null = {<No data fields>}, static shared_null = {ref = {_q_value = 10222}, alloc = 0,
    size = 0, data = 0xb652dfba, clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0,
    reserved = 0, array = {0}}, static shared_empty = {ref = {_q_value = 501}, alloc = 0, size = 0,
    data = 0xb652dfce, clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0,
    array = {0}}, d = 0xb652dfa8, static codecForCStrings = 0x0}
        u = {<QUrl> = {d = 0xa52c170}, d = 0x0}
       pathCombo = (class KUrlComboBox *) 0xa4d2d00
        goToNavigatorAction = (class KAction *) 0xbfee0f4c
        viewConfigGroup = {<KConfigBase> = {_vptr.KConfigBase = 0xb73d96a8}, d = {d = 0xa548e40}}
        docPath = {<QUrl> = {d = 0xa52c110}, d = 0x0}
        coll = <value optimized out>
        whatsThisText = {static null = {<No data fields>}, static shared_null = {ref = {_q_value = 10222},
    alloc = 0, size = 0, data = 0xb652dfba, clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0,
    capacity = 0, reserved = 0, array = {0}}, static shared_empty = {ref = {_q_value = 501}, alloc = 0, size = 0,
    data = 0xb652dfce, clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0,
    array = {0}}, d = 0xa543688, static codecForCStrings = 0x0}
        config = {d = 0x9d077b0}
        pg = <value optimized out>
        statJob = (class KIO::StatJob *) 0xa54e4a8
#13 0xa865a40b in KFileModule::createFileWidget (this=0xa1ce568, startDir=@0xbfee124c, parent=0xa1b8040)
    at /home/jan/kdemod/core/kdelibs/src/kdelibs-4.2.4/kfile/kfilemodule.cpp:36
No locals.
#14 0xb7b2f815 in KFileDialog (this=0xa1b8040, startDir=@0xbfee124c, filter=@0xbfee125c, parent=0xa1b7910,
    customWidget=0x0) at /home/jan/kdemod/core/kdelibs/src/kdelibs-4.2.4/kio/kfile/kfiledialog.cpp:251
        fileQWidget = <value optimized out>
#15 0xa86f70b5 in Image::showFileDialog () from /usr/lib/kde4/plasma_wallpaper_image.so
No symbol table info available.
#16 0xa86f8983 in Image::qt_metacall () from /usr/lib/kde4/plasma_wallpaper_image.so
No symbol table info available.
#17 0xb645381c in QMetaObject::activate () from /usr/lib/libQtCore.so.4
No symbol table info available.
#18 0xb6453c68 in QMetaObject::activate () from /usr/lib/libQtCore.so.4
No symbol table info available.
#19 0xb6d6f5f1 in QAbstractButton::clicked () from /usr/lib/libQtGui.so.4
No symbol table info available.
#20 0xb6a9bf39 in ?? () from /usr/lib/libQtGui.so.4
No symbol table info available.
#21 0xb6a9db94 in ?? () from /usr/lib/libQtGui.so.4
No symbol table info available.
#22 0xb6a9de21 in QAbstractButton::mouseReleaseEvent () from /usr/lib/libQtGui.so.4
No symbol table info available.
#23 0xb6b8333c in QToolButton::mouseReleaseEvent () from /usr/lib/libQtGui.so.4
No symbol table info available.
#24 0xb6714e52 in QWidget::event () from /usr/lib/libQtGui.so.4
No symbol table info available.
#25 0xb6a9bdde in QAbstractButton::event () from /usr/lib/libQtGui.so.4
No symbol table info available.
#26 0xb6b85cea in QToolButton::event () from /usr/lib/libQtGui.so.4
#27 0xb66c0554 in QApplicationPrivate::notify_helper () from /usr/lib/libQtGui.so.4
No symbol table info available.
#28 0xb66c8590 in QApplication::notify () from /usr/lib/libQtGui.so.4
No symbol table info available.
#29 0xb75bc81a in KApplication::notify (this=0x9d081e0, receiver=0xa1b7ee0, event=0xbfee1ac4)
    at /home/jan/kdemod/core/kdelibs/src/kdelibs-4.2.4/kdeui/kernel/kapplication.cpp:307
No locals.
#30 0xb643db4b in QCoreApplication::notifyInternal () from /usr/lib/libQtCore.so.4
No symbol table info available.
#31 0xb66c7553 in QApplicationPrivate::sendMouseEvent () from /usr/lib/libQtGui.so.4
No symbol table info available.
#32 0xb6734f34 in ?? () from /usr/lib/libQtGui.so.4
No symbol table info available.
#33 0xb6734415 in QApplication::x11ProcessEvent () from /usr/lib/libQtGui.so.4
No symbol table info available.
#34 0xb675e982 in ?? () from /usr/lib/libQtGui.so.4
No symbol table info available.
#35 0xb4d64d98 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#36 0xb4d683e0 in g_main_context_iterate () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#37 0xb4d68513 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#38 0xb64684ec in QEventDispatcherGlib::processEvents () from /usr/lib/libQtCore.so.4
No symbol table info available.
#39 0xb675e0b5 in ?? () from /usr/lib/libQtGui.so.4
No symbol table info available.
#40 0xb643c0f9 in QEventLoop::processEvents () from /usr/lib/libQtCore.so.4
No symbol table info available.
#41 0xb643c542 in QEventLoop::exec () from /usr/lib/libQtCore.so.4
No symbol table info available.
#42 0xb643e89f in QCoreApplication::exec () from /usr/lib/libQtCore.so.4
No symbol table info available.
#43 0xb66c03d7 in QApplication::exec () from /usr/lib/libQtGui.so.4
No symbol table info available.
#44 0xb7f5cb76 in kdemain () from /usr/lib/libkdeinit4_plasma.so
No symbol table info available.
#45 0x0804871b in _start ()
No symbol table info available.

This looks related to bug 193176. Instead of using GDB, could you try to run "kquitapp plasma-desktop" and see if it also crashes with a similar backtrace ?
Thanks

Yep, it gives a similar backtrace. 

I installed 4.3.0 RC3 and the problem seems to have been fixed, so I would count this as solved :)

I am afraid its back.. still exists in 4.3, seems to work sometimes and sometimes not at all :(

I did kquitapp plasma-desktop as you asked, backtrace as follows:

Application: Plasma Workspace (kdeinit4), signal: Segmentation fault
[KCrash Handler]
#5  0x00007f0b98129d8b in QDBusAdaptorConnector::relaySlot (this=0x7f0b740203f0, argv=0x7fff8a43cfa0) at qdbusabstractadaptor.cpp:268
#6  0x00007f0b9812a62a in QDBusAdaptorConnector::qt_metacall (this=0x7f0b740203f0, _c=QMetaObject::InvokeMetaMethod, _id=<value optimized out>, _a=0x7fff8a43cfa0) at qdbusabstractadaptor.cpp:364
#7  0x00007f0b9c10d841 in QMetaObject::activate (sender=<value optimized out>, from_signal_index=<value optimized out>, to_signal_index=<value optimized out>, argv=0x7fff8a43cfa0)
    at kernel/qobject.cpp:3112
#8  0x00007f0b9c10dcd4 in QObject::destroyed (this=0x7f0b740203f0, _t1=0x7f0b740252c0) at .moc/debug-shared/moc_qobject.cpp:143
#9  0x00007f0b9c10ec6b in ~QObject (this=<value optimized out>) at kernel/qobject.cpp:757
#10 0x00007f0b9b04c7a7 in KIO::Scheduler::~Scheduler () from /usr/lib/libkio.so.5
#11 0x00007f0b9b04ff91 in ._259::destroy () from /usr/lib/libkio.so.5
#12 0x00007f0b9962e9e2 in __run_exit_handlers () from /lib/libc.so.6
#13 0x00007f0b9962ea35 in exit () from /lib/libc.so.6
#14 0x0000000000406eb5 in _start ()

More backtrace info:

Application: Plasma Workspace (kdeinit4), signal: Aborted
[Current thread is 0 (LWP 27183)]

Thread 3 (Thread 0x7f7b83114910 (LWP 27186)):
#0  0x00007f7b9fc38a6b in g_main_context_check () from /usr/lib/libglib-2.0.so.0
#1  0x00007f7b9fc39260 in g_main_context_iterate () from /usr/lib/libglib-2.0.so.0
#2  0x00007f7b9fc39690 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#3  0x00007f7ba300755e in QEventDispatcherGlib::processEvents (this=0x107c1a0, flags=<value optimized out>) at kernel/qeventdispatcher_glib.cpp:329
#4  0x00007f7ba2fda4b2 in QEventLoop::processEvents (this=<value optimized out>, flags=) at kernel/qeventloop.cpp:149
#5  0x00007f7ba2fda884 in QEventLoop::exec (this=0x7f7b83110460, flags=) at kernel/qeventloop.cpp:201
#6  0x00007f7ba1f2221c in KIO::NetAccess::enter_loop () from /usr/lib/libkio.so.5
#7  0x00007f7ba1f2286f in KIO::NetAccess::synchronousRunInternal () from /usr/lib/libkio.so.5
#8  0x00007f7ba1f22ef1 in KIO::NetAccess::synchronousRun () from /usr/lib/libkio.so.5
#9  0x00007f7b86b41915 in Update_Thread::downloadFile () from /usr/lib/kde4/plasma_applet_cwp.so
#10 0x00007f7b86b4c6ee in Update_Thread::collectAndParseData () from /usr/lib/kde4/plasma_applet_cwp.so
#11 0x00007f7b86b4dbf8 in Update_Thread::run () from /usr/lib/kde4/plasma_applet_cwp.so
#12 0x00007f7ba2ee9305 in QThreadPrivate::start (arg=0x105d890) at thread/qthread_unix.cpp:188
#13 0x00007f7ba2c7257a in start_thread () from /lib/libpthread.so.0
#14 0x00007f7ba05ab16d in clone () from /lib/libc.so.6
#15 0x0000000000000000 in ?? ()

Thread 2 (Thread 0x7f7b825c4910 (LWP 27188)):
#0  0x00007f7ba2c7705d in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib/libpthread.so.0
#1  0x00007f7ba2eea512 in QWaitCondition::wait (this=<value optimized out>, mutex=0x1592d00, time=30000) at thread/qwaitcondition_unix.cpp:85
#2  0x00007f7ba2edf19a in QThreadPoolThread::run (this=<value optimized out>) at concurrent/qthreadpool.cpp:140
#3  0x00007f7ba2ee9305 in QThreadPrivate::start (arg=0x1592e40) at thread/qthread_unix.cpp:188
#4  0x00007f7ba2c7257a in start_thread () from /lib/libpthread.so.0
#5  0x00007f7ba05ab16d in clone () from /lib/libc.so.6
#6  0x0000000000000000 in ?? ()

Thread 1 (Thread 0x7f7ba34c3760 (LWP 27183)):
[KCrash Handler]
#5  0x00007f7ba0511f15 in raise () from /lib/libc.so.6
#6  0x00007f7ba0513340 in abort () from /lib/libc.so.6
#7  0x00007f7b863d6f49 in bt::QtMessageOutput () from /usr/lib/libbtcore.so.11
#8  0x00007f7ba2ee017e in qt_message_output (msgType=QtFatalMsg, buf=0x6a2f <Address 0x6a2f out of bounds>) at global/qglobal.cpp:2007
#9  0x00007f7ba2ee0316 in qFatal (msg=<value optimized out>) at global/qglobal.cpp:2241
#10 0x00007f7b9efee2e9 in QDBusConnectionPrivate::connectSignal (this=<value optimized out>, key=<value optimized out>, hook=@0x7fff567debe0) at qdbusintegrator.cpp:1949
#11 0x00007f7b9eff0f32 in QDBusConnectionPrivate::connectRelay (this=0xb9f6d0, service=<value optimized out>, owner=<value optimized out>, path=<value optimized out>, interface=@0xba59d0, 
    receiver=<value optimized out>, signal=0x7f7b9f02b190 "2NameAcquired(QString)") at qdbusintegrator.cpp:2036
#12 0x00007f7b9effde06 in QDBusAbstractInterface::connectNotify (this=0xba0770, signal=0x7f7b9f02b190 "2NameAcquired(QString)") at qdbusabstractinterface.cpp:516
#13 0x00007f7ba2fef032 in QObject::connect (sender=0xba0770, signal=<value optimized out>, receiver=0x158c180, method=<value optimized out>, type=Qt::AutoConnection) at kernel/qobject.cpp:2547
#14 0x00007f7b8270a7a5 in ktplasma::Engine::Engine () from /usr/lib/kde4/plasma_engine_ktorrent.so
#15 0x00007f7b8270b597 in KPluginFactory::createInstance<ktplasma::Engine, QObject> () from /usr/lib/kde4/plasma_engine_ktorrent.so
#16 0x00007f7ba29dbdde in KPluginFactory::create () from /usr/lib/libkdecore.so.5
#17 0x00007f7b9a92ef26 in Plasma::DataEngineManager::loadEngine () from /usr/lib/libplasma.so.3
#18 0x00007f7b9a9010a0 in Plasma::Applet::dataEngine () from /usr/lib/libplasma.so.3
#19 0x00007f7b8691e010 in ktplasma::Applet::init () from /usr/lib/kde4/plasma_applet_ktorrent.so
#20 0x00007f7b9a9261aa in Plasma::Corona::loadLayout () from /usr/lib/libplasma.so.3
#21 0x00007f7b9a927252 in Plasma::Corona::initializeLayout () from /usr/lib/libplasma.so.3
#22 0x00007f7b96e3a986 in PlasmaApp::corona () from /usr/lib/libkdeinit4_plasma-desktop.so
#23 0x00007f7b96e3ac58 in PlasmaApp::setupDesktop () from /usr/lib/libkdeinit4_plasma-desktop.so
#24 0x00007f7b96e3ae6c in PlasmaApp::qt_metacall () from /usr/lib/libkdeinit4_plasma-desktop.so
#25 0x00007f7ba2ff3841 in QMetaObject::activate (sender=<value optimized out>, from_signal_index=<value optimized out>, to_signal_index=<value optimized out>, argv=0xffffffffffffffff)
    at kernel/qobject.cpp:3112
#26 0x00007f7ba2ff8f1f in QSingleShotTimer::timerEvent (this=0xc6ef10) at kernel/qtimer.cpp:298
#27 0x00007f7ba2fec2d3 in QObject::event (this=0xc6ef10, e=0x6a2f) at kernel/qobject.cpp:1074
#28 0x00007f7ba0ceabec in QApplicationPrivate::notify_helper (this=0xba7170, receiver=0xc6ef10, e=0x7fff567e00c0) at kernel/qapplication.cpp:4056
#29 0x00007f7ba0cf1f6d in QApplication::notify (this=0xb9e010, receiver=0xc6ef10, e=0x7fff567e00c0) at kernel/qapplication.cpp:4021
#30 0x00007f7ba199aff6 in KApplication::notify () from /usr/lib/libkdeui.so.5
#31 0x00007f7ba2fdba9c in QCoreApplication::notifyInternal (this=0xb9e010, receiver=0xc6ef10, event=0x7fff567e00c0) at kernel/qcoreapplication.cpp:610
#32 0x00007f7ba300ab1a in QTimerInfoList::activateTimers (this=0xbaa6e0) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:213
#33 0x00007f7ba30075bd in timerSourceDispatch (source=<value optimized out>) at kernel/qeventdispatcher_glib.cpp:165
#34 0x00007f7b9fc35dbe in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#35 0x00007f7b9fc39568 in g_main_context_iterate () from /usr/lib/libglib-2.0.so.0
#36 0x00007f7b9fc39690 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#37 0x00007f7ba3007506 in QEventDispatcherGlib::processEvents (this=0xaef5a0, flags=<value optimized out>) at kernel/qeventdispatcher_glib.cpp:327
#38 0x00007f7ba0d860be in QGuiEventDispatcherGlib::processEvents (this=0x6a2f, flags=<value optimized out>) at kernel/qguieventdispatcher_glib.cpp:202
#39 0x00007f7ba2fda4b2 in QEventLoop::processEvents (this=<value optimized out>, flags=) at kernel/qeventloop.cpp:149
#40 0x00007f7ba2fda884 in QEventLoop::exec (this=0x7fff567e0360, flags=) at kernel/qeventloop.cpp:201
#41 0x00007f7ba2fdcc4b in QCoreApplication::exec () at kernel/qcoreapplication.cpp:888
#42 0x00007f7b96e211ff in kdemain () from /usr/lib/libkdeinit4_plasma-desktop.so
#43 0x0000000000406eae in _start ()

Actually i lie, that last bug comment is from when you logout with ktorrent and the torrent plasma widget on at the same time. I shall open a new bug report for that!

Managed to get a better backtrace:

Application: Plasma Workspace (kdeinit4), signal: Segmentation fault
[Current thread is 0 (LWP 2569)]

Thread 4 (Thread 0x7f0186034910 (LWP 2570)):
[KCrash Handler]
#5  0x00007f01a61231e0 in QObject::disconnect (sender=<value optimized out>, signal=<value optimized out>, receiver=0x2a98370, method=<value optimized out>) at kernel/qobject.cpp:2672
#6  0x00007f01a50659b3 in KIO::SchedulerPrivate::slotUnregisterWindow () from /usr/lib/libkio.so.5
#7  0x00007f01a50691df in KIO::Scheduler::qt_metacall () from /usr/lib/libkio.so.5
#8  0x00007f01a611c649 in QObject::event (this=0x2a98370, e=0x3828060) at kernel/qobject.cpp:1110
#9  0x00007f01a3e1abec in QApplicationPrivate::notify_helper (this=0x21330a0, receiver=0x2a98370, e=0x3828060) at kernel/qapplication.cpp:4056
#10 0x00007f01a3e21f6d in QApplication::notify (this=0x2129f40, receiver=0x2a98370, e=0x3828060) at kernel/qapplication.cpp:4021
#11 0x00007f01a4acaff6 in KApplication::notify () from /usr/lib/libkdeui.so.5
#12 0x00007f01a610ba9c in QCoreApplication::notifyInternal (this=0x2129f40, receiver=0x2a98370, event=0x3828060) at kernel/qcoreapplication.cpp:610
#13 0x00007f01a610c954 in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=<value optimized out>, data=0x25e42c0)
    at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:213
#14 0x00007f01a613ac20 in QEventDispatcherUNIX::processEvents (this=0x2a9de80, flags=) at kernel/qeventdispatcher_unix.cpp:876
#15 0x00007f01a610a4b2 in QEventLoop::processEvents (this=<value optimized out>, flags=) at kernel/qeventloop.cpp:149
#16 0x00007f01a610a884 in QEventLoop::exec (this=0x7f0186030460, flags=) at kernel/qeventloop.cpp:201
#17 0x00007f01a505221c in KIO::NetAccess::enter_loop () from /usr/lib/libkio.so.5
#18 0x00007f01a505286f in KIO::NetAccess::synchronousRunInternal () from /usr/lib/libkio.so.5
#19 0x00007f01a5052ef1 in KIO::NetAccess::synchronousRun () from /usr/lib/libkio.so.5
#20 0x00007f0188359915 in Update_Thread::downloadFile () from /usr/lib/kde4/plasma_applet_cwp.so
#21 0x00007f0188361e13 in Update_Thread::collectAndParseData () from /usr/lib/kde4/plasma_applet_cwp.so
#22 0x00007f0188365bf8 in Update_Thread::run () from /usr/lib/kde4/plasma_applet_cwp.so
#23 0x00007f01a6019305 in QThreadPrivate::start (arg=0x26fde70) at thread/qthread_unix.cpp:188
#24 0x00007f01a5da257a in start_thread () from /lib/libpthread.so.0
#25 0x00007f01a36db16d in clone () from /lib/libc.so.6
#26 0x0000000000000000 in ?? ()

Thread 3 (Thread 0x7f017f7c7910 (LWP 2574)):
#0  0x00007f01a5da6dd9 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib/libpthread.so.0
#1  0x00007f01a601a5fb in QWaitCondition::wait (this=<value optimized out>, mutex=0x2ed0a18, time=18446744073709551615) at thread/qwaitcondition_unix.cpp:87
#2  0x00007f01a23f9e24 in QHostInfoAgent::run (this=0x2ed0a00) at kernel/qhostinfo.cpp:260
#3  0x00007f01a6019305 in QThreadPrivate::start (arg=0x2ed0a00) at thread/qthread_unix.cpp:188
#4  0x00007f01a5da257a in start_thread () from /lib/libpthread.so.0
#5  0x00007f01a36db16d in clone () from /lib/libc.so.6
#6  0x0000000000000000 in ?? ()

Thread 2 (Thread 0x7f0184ece910 (LWP 3845)):
#0  0x00007f01a5da705d in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib/libpthread.so.0
#1  0x00007f01a601a512 in QWaitCondition::wait (this=<value optimized out>, mutex=0x2c6fc80, time=30000) at thread/qwaitcondition_unix.cpp:85
#2  0x00007f01a600f19a in QThreadPoolThread::run (this=<value optimized out>) at concurrent/qthreadpool.cpp:140
#3  0x00007f01a6019305 in QThreadPrivate::start (arg=0x2c62950) at thread/qthread_unix.cpp:188
#4  0x00007f01a5da257a in start_thread () from /lib/libpthread.so.0
#5  0x00007f01a36db16d in clone () from /lib/libc.so.6
#6  0x0000000000000000 in ?? ()

Thread 1 (Thread 0x7f01a65f3760 (LWP 2569)):
#0  0x00007f01a5da4ad0 in pthread_mutex_lock () from /lib/libpthread.so.0
#1  0x00007f01a1872917 in _xcb_conn_wait () from /usr/lib/libxcb.so.1
#2  0x00007f01a1872e37 in _xcb_out_send () from /usr/lib/libxcb.so.1
#3  0x00007f01a18730e5 in xcb_writev () from /usr/lib/libxcb.so.1
#4  0x00007f01a53e07aa in _XSend () from /usr/lib/libX11.so.6
#5  0x00007f01a53e08e9 in _XReply () from /usr/lib/libX11.so.6
#6  0x00007f01a53bd6e5 in XGetImage () from /usr/lib/libX11.so.6
#7  0x00007f01a3ef3b9b in QX11PixmapData::toImage (this=0x3761fd0) at image/qpixmap_x11.cpp:1428
#8  0x00007f01a3ee372d in QPixmap::toImage (this=0x35d32c8) at image/qpixmap.cpp:447
#9  0x00007f01a4b44d83 in KPixmapCache::Private::writeData () from /usr/lib/libkdeui.so.5
#10 0x00007f01a4b4599b in KPixmapCache::insert () from /usr/lib/libkdeui.so.5
#11 0x00007f019dab1d68 in Plasma::ThemePrivate::scheduledCacheUpdate () from /usr/lib/libplasma.so.3
#12 0x00007f019dab69ea in Plasma::Theme::qt_metacall () from /usr/lib/libplasma.so.3
#13 0x00007f01a6123841 in QMetaObject::activate (sender=<value optimized out>, from_signal_index=<value optimized out>, to_signal_index=<value optimized out>, argv=0x7f01a36d29b3)
    at kernel/qobject.cpp:3112
#14 0x00007f01a611c2d3 in QObject::event (this=0x222ed00, e=0x0) at kernel/qobject.cpp:1074
#15 0x00007f01a3e1abec in QApplicationPrivate::notify_helper (this=0x21330a0, receiver=0x222ed00, e=0x7fff5cb280c0) at kernel/qapplication.cpp:4056
#16 0x00007f01a3e21f6d in QApplication::notify (this=0x2129f40, receiver=0x222ed00, e=0x7fff5cb280c0) at kernel/qapplication.cpp:4021
#17 0x00007f01a4acaff6 in KApplication::notify () from /usr/lib/libkdeui.so.5
#18 0x00007f01a610ba9c in QCoreApplication::notifyInternal (this=0x2129f40, receiver=0x222ed00, event=0x7fff5cb280c0) at kernel/qcoreapplication.cpp:610
#19 0x00007f01a613ab1a in QTimerInfoList::activateTimers (this=0x2133b60) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:213
#20 0x00007f01a613ac9c in QEventDispatcherUNIX::processEvents (this=0x20775a0, flags=) at kernel/qeventdispatcher_unix.cpp:908
#21 0x00007f01a3eb7317 in QEventDispatcherX11::processEvents (this=<value optimized out>, flags=<value optimized out>) at kernel/qeventdispatcher_x11.cpp:152
#22 0x00007f01a610a4b2 in QEventLoop::processEvents (this=<value optimized out>, flags=) at kernel/qeventloop.cpp:149
#23 0x00007f01a610a884 in QEventLoop::exec (this=0x7fff5cb285a0, flags=) at kernel/qeventloop.cpp:201
#24 0x00007f01a610cc4b in QCoreApplication::exec () at kernel/qcoreapplication.cpp:888
#25 0x00007f0199f511ff in kdemain () from /usr/lib/libkdeinit4_plasma-desktop.so
#26 0x0000000000406eae in _start ()

Mh, now I'm confused: the last backtrace seems more from a crash than from a freeze.. Did you get it by using the same steps as comment 0?
Crash in comment 7 could be related to a Qt-Dbus issue (I think it was reported too)
Regards

Sorry, I sometimes get the comment 9 crash when I click the browse background button and after the time-out it crashes (its about a 2 minute timeout).. this only happens sometimes tho, sometimes it just opens the file dialog with no files viewable.

Other filedialog's work fine, its just ones with plasma-desktop. I don't know if this could be the plasma-desktop qtbus issue but looking the code in kdebase/worksapce/plasma/wallpaper/image the filedialog code looks fine to me. 

Comment 7 is a ktorrent qt-dbus issue I believe, I have reported it in a separate bug report.

Merging with bug 193176. Thanks

*** This bug has been marked as a duplicate of bug 193176 ***