Version:           revision 779292 (4.0 branch) (using Devel)
Installed from:    Compiled sources
Compiler:          gcc (GCC) 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2) 

When I start a KDE4 session, plasma always crashes. Sometimes I get a (totally non-functioning) white Desktop with a Panel saying something like "This object could not be loaded", but usually the background of the log-in screen stays. In both cases all I can do is Ctrl+Alt+Backspace.
I'm using a self-compiled Qt 4.3.4 (vanilla, not qt-copy), as well as kdelibs, kdepimlib and kdebase from KDE 4.0 branch. The rest of the dependencies is provided by my distribution (Kubuntu 7.10).
Here's the backtrace of plasma:

Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".
[Thread debugging using libthread_db enabled]
[New Thread -1249781504 (LWP 12125)]
[KCrash handler]
#6  0xb869e59e in ?? ()
#7  0xb7ea518c in Plasma::Applet::flushUpdatedConstraints (this=0x8102978)
    at /home/kde4/kde/src/KDE/kdebase/workspace/libs/plasma/applet.cpp:699
#8  0xb7ed1879 in Plasma::Corona::loadDefaultSetup (this=0x80ea058)
    at /home/kde4/kde/src/KDE/kdebase/workspace/libs/plasma/corona.cpp:315
#9  0xb7ed0fef in Plasma::Corona::loadApplets (this=0x80ea058, 
    configName=@0x80f0ff0)
    at /home/kde4/kde/src/KDE/kdebase/workspace/libs/plasma/corona.cpp:258
#10 0xb7ed14f7 in Plasma::Corona::loadApplets (this=0x80ea058)
    at /home/kde4/kde/src/KDE/kdebase/workspace/libs/plasma/corona.cpp:296
#11 0xb7fa9eec in PlasmaApp::corona (this=0x8075948)
    at /home/kde4/kde/src/KDE/kdebase/workspace/plasma/plasma/plasmaapp.cpp:219
#12 0xb7facbaa in RootWidget (this=0x80e9cb0)
    at /home/kde4/kde/src/KDE/kdebase/workspace/plasma/plasma/rootwidget.cpp:49
#13 0xb7faa8e6 in PlasmaApp (this=0x8075948, display=0x805b070, 
    visual=134613048, colormap=25165825)
    at /home/kde4/kde/src/KDE/kdebase/workspace/plasma/plasma/plasmaapp.cpp:156
#14 0xb7fab079 in PlasmaApp::self ()
    at /home/kde4/kde/src/KDE/kdebase/workspace/plasma/plasma/plasmaapp.cpp:96
#15 0xb7fa98ec in kdemain (argc=1, argv=0xbffab304)
    at /home/kde4/kde/src/KDE/kdebase/workspace/plasma/plasma/main.cpp:51
#16 0x08048792 in main (argc=1470184, argv=0xb7fb5c50)
    at /home/kde4/kde/build/KDE/kdebase/workspace/plasma/plasma/plasma-qgv_dummy.cpp:3
#0  0xffffe410 in __kernel_vsyscall ()


However, I've got the impression that the problem is somehow related to loading plugins: When I manage to start Konqueror from the crash dialogue, it complains about not finding the KHTML component. However when I start KDE4 Konqueror from within my KDE3 session, it works fine. Starting plasma within the KDE3 session also succeeds, I get a 2nd panel ;-)
As I played around with the environment variables to get it working at least in KDE3 sessions, I guess it might be related to them... here's the part of my .bashrc which sets up the KDE4 environment:

# KDE
export KDEDIR=$HOME/kde
export KDEHOME=$HOME/.kde4
export KDETMP=/tmp/$USER-kde4
mkdir -p $KDETMP
export KDEDIRS=$KDEDIR
export PKG_CONFIG_PATH=$KDEDIR/lib/pkgconfig

# Qt
# only set Qt related variables if you compiled Qt on your own
# (which is discouraged). if you use the distro provided Qt, skip
# this section. Comment it if neccessary.
export QTDIR=/opt/qt-4.3.4
export QT_PLUGIN_PATH=$KDEDIR/lib/kde4/plugins
export PKG_CONFIG_PATH=$QTDIR/lib/pkgconfig:$PKG_CONFIG_PATH

# Other
# you might want to remove $PATH from the end of this line and add any
# essential items from your path - to make sure the your kde3 path isn't
# in here. Use the commented versions if you installed Qt through your distro instead of qt-copy.
export PATH=$KDEDIR/bin:$PATH
export LD_LIBRARY_PATH=$KDEDIR/lib:$QTDIR/lib

# XDG
unset XDG_DATA_DIRS # to avoid seeing kde3 files from /usr
unset XDG_CONFIG_DIRS

# you might want to change these:
export KDE_BUILD=$HOME/kde/build
export KDE_SRC=$HOME/kde/src

# Make
# Tell many scripts how to switch from source dir to build dir:
export OBJ_REPLACEMENT="s#$KDE_SRC#$KDE_BUILD#"