It's a new 21.5" Late 2011 base model running Lion. I didn't start doing this until this afternoon, after I installed additional RAM. I'm assuming this has to be related to it somehow. I installed a 2x4GB set (in addition to the standard 2x2) for a total of 12GB in the system. In System Profiler, it's showing the correct amount installed. However, whenever I go to put the machine to sleep, it'll 'sleep' for a few moments, then do a full restart (not just wake from sleep). I tried shutting down the machine, and that worked. However, even after a full shutdown, the sleep issue still persists. I tried resetting the PRAM but the problem still exists.