I'm also suffering from this problem.
I'm running Ubuntu 12.04, with an Nvidia Geforce 320M. I've generated the nvidia-bug-report.log.gz file, but can't attach because the log file is too big.
In any case, I've read the forums and can't see any direct link to the issue on Launchpad, so thought I would attach a link just in case.
I'm running Unity 2D. The problem occurs when using Flash, VMD, Vdpau (through Mythtv or Gnome Mplayer) and sometimes seems to be completely without cause. I've tried Nvidia versions 295.33, 295.40 and 295.49 without any success. I'm running Kernel version 18.104.22.168 and X Server version 1.11.3.
Note the issue does not occur immediately, it seems slightly random, but definately happens at least once, and sometimes multiple times a day.