Version:            (using KDE 4.3.4)
Compiler:          gcc 4.4.2 
OS:                Linux
Installed from:    Gentoo Packages

HAL Crashes with the following backtrace:

Thread 1 (Thread 0xb468b930 (LWP 15537)):
[KCrash Handler]
#6  qNumVariantToHelper<double> (this=0x0, ok=0x0) at kernel/qvariant.cpp:2361
#7  QVariant::toDouble (this=0x0, ok=0x0) at kernel/qvariant.cpp:2475
#8  0xa8f832ca in HalPower::brightness (this=0x8678918, device=...) at /var/tmp/portage/kde-base/solid-4.3.4/work/solid-4.3.4/solid/hal/halpower.cpp:396
#9  0xa928ef26 in Solid::Control::PowerManager::brightness (device=...) at /var/tmp/portage/kde-base/solid-4.3.4/work/solid-4.3.4/libs/solid/control/powermanager.cpp:198
#10 0xa903f155 in ?? () from /usr/lib/kde4/plasma_applet_battery.so
#11 0xbfd5e07c in ?? ()
#12 0x00000000 in ?? ()

Steps to reproduce:

Start KDE with battery plasmoid in panel on Thinkpad T60p. The crash happens immediately. Alternatively start plasma-desktop.

My environment is:
Thinkpad T60p, Gentoo, Kernel 2.6.32-gentoo (sys-kernel/gentoo-sources-2.6.32), sys-apps/hal-0.5.14, qt 4.6

Quick analysis:

This is the source from halpower.cpp

    if(m_halManager.call("FindDeviceByCapability", "keyboard_backlight").arguments().at(0).toStringList().contains(device))
    {
        QDBusInterface deviceInterface("org.freedesktop.Hal", device, "org.freedesktop.Hal.Device.KeyboardBacklight", QDBusConnection::systemBus()); //TODO - I do not have a backlight enabled keyboard, so I'm guessing a bit here. Could someone please check this.
        brightness = deviceInterface.call("GetBrightness").arguments().at(0).toDouble();
        if(deviceInterface.lastError().isValid())
        {
            return 0;
        }
        else
        {
            QDBusInterface propertyInterface("org.freedesktop.Hal", device, "org.freedesktop.Hal.Device", QDBusConnection::systemBus());
            int levels = propertyInterface.call("GetProperty", "keyboard_backlight.num_levels").arguments().at(0).toInt();
            return (float)(100*(brightness/(levels-1)));
        }
    }
    return 0;


The line after the comment "TODO - ..." is the one that crashes because at(0) returns 0.

This is what HAL tells me on the console.

~ > hal-find-by-capability --capability keyboard_backlight
/org/freedesktop/Hal/devices/leds_tpacpi_thinkvantage
/org/freedesktop/Hal/devices/leds_tpacpi_thinklight
/org/freedesktop/Hal/devices/leds_tpacpi_standby
/org/freedesktop/Hal/devices/leds_tpacpi_power

~ > hal-device
[...]
24: udi = '/org/freedesktop/Hal/devices/leds_tpacpi_thinkvantage'
  linux.hotplug_type = 2  (0x2)  (int)
  linux.subsystem = 'leds'  (string)
  info.capabilities = { 'leds', 'keyboard_backlight' } (string list)
  info.category = 'leds'  (string)
  info.interfaces = { 'org.freedesktop.Hal.Device.Leds' } (string list)
  info.subsystem = 'leds'  (string)
  linux.sysfs_path = '/sys/devices/platform/thinkpad_acpi/leds/tpacpi::thinkvantage'  (string)
  info.parent = '/org/freedesktop/Hal/devices/platform_thinkpad_acpi'  (string)
  leds.device_name = 'tpacpi'  (string)
  keyboard_backlight.num_levels = 256  (0x100)  (int)
  leds.num_levels = 256  (0x100)  (int)
  leds.function = 'thinkvantage'  (string)
  info.udi = '/org/freedesktop/Hal/devices/leds_tpacpi_thinkvantage'  (string)
  info.addons.singleton = { 'hald-addon-leds' } (string list)

25: udi = '/org/freedesktop/Hal/devices/leds_tpacpi_thinklight'
  linux.hotplug_type = 2  (0x2)  (int)
  linux.subsystem = 'leds'  (string)
  info.capabilities = { 'leds', 'keyboard_backlight' } (string list)
  info.category = 'leds'  (string)
  info.interfaces = { 'org.freedesktop.Hal.Device.Leds' } (string list)
  info.subsystem = 'leds'  (string)
  linux.sysfs_path = '/sys/devices/platform/thinkpad_acpi/leds/tpacpi::thinklight'  (string)
  info.parent = '/org/freedesktop/Hal/devices/platform_thinkpad_acpi'  (string)
  leds.device_name = 'tpacpi'  (string)
  keyboard_backlight.num_levels = 256  (0x100)  (int)
  leds.num_levels = 256  (0x100)  (int)
  leds.function = 'thinklight'  (string)
  info.udi = '/org/freedesktop/Hal/devices/leds_tpacpi_thinklight'  (string)
  info.addons.singleton = { 'hald-addon-leds' } (string list)

26: udi = '/org/freedesktop/Hal/devices/leds_tpacpi_standby'
  linux.hotplug_type = 2  (0x2)  (int)
  linux.subsystem = 'leds'  (string)
  info.capabilities = { 'leds', 'keyboard_backlight' } (string list)
  info.category = 'leds'  (string)
  info.interfaces = { 'org.freedesktop.Hal.Device.Leds' } (string list)
  info.subsystem = 'leds'  (string)
  linux.sysfs_path = '/sys/devices/platform/thinkpad_acpi/leds/tpacpi::standby'  (string)
  info.parent = '/org/freedesktop/Hal/devices/platform_thinkpad_acpi'  (string)
  leds.device_name = 'tpacpi'  (string)
  keyboard_backlight.num_levels = 256  (0x100)  (int)
  leds.num_levels = 256  (0x100)  (int)
  leds.function = 'standby'  (string)
  info.udi = '/org/freedesktop/Hal/devices/leds_tpacpi_standby'  (string)
  info.addons.singleton = { 'hald-addon-leds' } (string list)

27: udi = '/org/freedesktop/Hal/devices/leds_tpacpi_power'
  linux.hotplug_type = 2  (0x2)  (int)
  linux.subsystem = 'leds'  (string)
  info.capabilities = { 'leds', 'keyboard_backlight' } (string list)
  info.category = 'leds'  (string)
  info.interfaces = { 'org.freedesktop.Hal.Device.Leds' } (string list)
  info.subsystem = 'leds'  (string)
  linux.sysfs_path = '/sys/devices/platform/thinkpad_acpi/leds/tpacpi::power'  (string)
  info.parent = '/org/freedesktop/Hal/devices/platform_thinkpad_acpi'  (string)
  leds.device_name = 'tpacpi'  (string)
  keyboard_backlight.num_levels = 256  (0x100)  (int)
  leds.num_levels = 256  (0x100)  (int)
  leds.function = 'power'  (string)
  info.udi = '/org/freedesktop/Hal/devices/leds_tpacpi_power'  (string)
  info.addons.singleton = { 'hald-addon-leds' } (string list)
[...]

This might be related to bug #209582 which is closed already, however.