I experience plasma crashing (without restarting itself) on Qt 4.4 and Qt 4.5.
The log of it looks like this:
kernel: plasma[5689] trap stack segment ip:7f8b94852c82 sp:7f8a7667bea0 error:0
At first i thought it was caused by some power management tools (i turned it off then) but it still happens. X server completely dies after that.
The funny thing is that i don’t even have to do anything and it crashes.
I installed the new 185.13 Beta Driver from nVidia and it didn’t crash for a while now so the nVidia driver may be the culprit.
Edit: Plasma crashed, still looking 
Found a solution. You need to delete the plasma files which are in /home/<user>/.kde4/share/config/ (plasmarc and there plasma-appletsrc). After that plasma works completely fine.
This behaviour usually occurs because of a buggy widget.
The hard part is sometime to figure out which widget is the root cause of crashes.