I get this reproducibly on every logon in KDE4; it seems to be restarted though,
so after some delay I still get desktop. Sometimes desktop is unusable (i.e.
applications react very sluggishly on mouse/button press, even if I can
start them via kicker or move around). Stack trace from plasma:

Application: Plasma Workspace (plasma), signal SIGSEGV
(no debugging symbols found)
Using host libthread_db library "/lib/i686/libthread_db.so.1".
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
[Thread debugging using libthread_db enabled]
[New Thread -1247582512 (LWP 12606)]
[New Thread -1293583472 (LWP 12648)]
[New Thread -1275991152 (LWP 12618)]
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
0xb809d424 in __kernel_vsyscall ()
[Current thread is 0 (LWP 12606)]

Thread 3 (Thread -1275991152 (LWP 12618)):
#0 0xb809d424 in __kernel_vsyscall ()
#1 0xb6c604e5 in pthread_cond_wait@@(protected) ()
 from /lib/i686/libpthread.so.0
#2 0xb6cdd8fe in QWaitCondition::wait () from /usr/lib/qt4/lib/libQtCore.so.4
#3 0xb430a46a in ?? () from /usr/lib/kde4/plasma_containment_desktop.so
#4 0x0810b52c in ?? ()
#5 0x0810b528 in ?? ()
#6 0xffffffff in ?? ()
#7 0x00000000 in ?? ()

Thread 2 (Thread -1293583472 (LWP 12648)):
#0 0xb809d424 in __kernel_vsyscall ()
#1 0xb6c604e5 in pthread_cond_wait@@(protected) ()
 from /lib/i686/libpthread.so.0
#2 0xb6cdd8fe in QWaitCondition::wait () from /usr/lib/qt4/lib/libQtCore.so.4
#3 0xb6bd5aa1 in ?? () from /usr/lib/qt4/lib/libQtNetwork.so.4
#4 0xb6cdcb59 in ?? () from /usr/lib/qt4/lib/libQtCore.so.4
#5 0xb6c5c4e2 in start_thread () from /lib/i686/libpthread.so.0
#6 0xb5de947e in clone () from /lib/i686/libc.so.6

Thread 1 (Thread -1247582512 (LWP 12606)):
#0 0xb809d424 in __kernel_vsyscall ()
#1 0xb5da9566 in nanosleep () from /lib/i686/libc.so.6
#2 0xb5da9347 in __sleep (seconds=0)
  at ../sysdeps/unix/sysv/linux/sleep.c:138
#3 0xb7c19813 in ?? () from /usr/lib/libkdeui.so.5
#4 0x00000001 in ?? ()
#5 0x00000000 in ?? ()
#0 0xb809d424 in __kernel_vsyscall ()