I'd like to add to this by saying that I have been able to upgrade 2011 iMac 27" Base i5-2400s processor to sandy bridge i7-2600k without any problems. It has been a year since the upgrade, and there have not been any problems to report (e.g. heat related, or power related issues). I would love to see someone try to put a ivy bridge processor inside the 2011 iMac to see if it would work. My feeling is that the EFI support and other kernel based tweaks would probably prevent it from working properly.