I recently installed kubuntu 11.04 on my laptop (system 76 pangolin 6 - nvidia graphics).. I frequently encounter lock ups when using the system. Example so far I have noticed -
1) start kopete or
2) start kdenlive and move it to the side of the window to resize in half (window snap)..or
3) System Settings -> window decorations -> choose a different window decorations and hit apply.

The only way recover seems to be to use REISUB.. I can not go to ctl+alt+f1 or any other terminal. I can ssh into the system - xorg is at 100% or kworker thread and xorg together make 100%. Any one seen these issues ?
Any help is greatly appreciated..
Thanks
ranj

Try disabling Desktop Effects and see if this continues.