I'm having the same problem.
Has anyone had any joy with a fix for this yet? Or is suffering too??
My 2011 imac is only a month old. I upgraded to 12GB RAM myself, and was running the preinstalled lion for a few weeks with no problems at all. Downloaded mountain lion, and noticed that the fans were either really quiet, or not kicking in at all. When I stream full screen video, the computer will randomly restart - telling me that there was a problem when the start-up gets going again. The top left hand corner was getting really hot, so i'd been using SMC to try and reduce the temperature thinking that this might have been the problem (SMU/PRAM reset had no impact), but last night it restarted twice seemingly regardless of the temp.
SO! Is there any chance that this is a mountain lion/software issue, or is it the logic board and the mountain lion upgrade timing was coincidence? If so, could it have been my installed RAM that killed it??
Any and all help/ideas appreciated. Next stop is obviously taking it to the genius bar, I just wanted to ask you clever people first.