I just got my iMac back from repair this evening and it shut itself down with a semi-loud POP and the system.log shows some
AppleSMU -- shutdown cause = -122 entries. I think it may be that OS X is trying to send light dimming instructions to the SMU, but the SMU's firmware hasn't been updated since the Apple genius put the new board into my iMac. I'm going to install the firmware update and hope this solves my problem. Otherwise, I'm looking at trip #3 to the Apple store for a problem they don't think exists. At least my CPU temps are down.
[edit]
Applied the light dimming firmware update and my iMac continues to spontaneously shut down. The Apple store won't take my iMac in for another look. Supposedly it passed their tests and I (as well as System.log) am imagining these problems.
Very frustrated but supposedly since I have AppleCare they said I could actually get onsite service since this will be repair call #3 on this iMac.