27 inch iMac "overheats"
The following happens for my 2010 266 GHz Intel Core i5 as well as an old white G4, both running 10.6.8 and tested with Temperature Monitor. When the Graphics Processor Temperature hits about 62C, the Mac shuts down consistently. Perhaps a sensor does so to protect the hardware, but 62C is not all that hot, far less than 85C or 100C. The only exception appears to be when a browser is in full-screen mode say when playing a Netflix movie, which is less than intuitive, also because of the hogging Silverlight plug-in.
iMac, Mac OS X (10.6.8)