For those of you who aren't afraid to take your Xbox 360 apart, and are having heat problems, pick up some Arctic Silver 5 thermal paste from your favorite computer parts store (www.newegg.com has some for a good price), and replace the thermal paste that's on both the processor and the GPU.
Most people, by the sound of it, are having GPU issues. ATi is known for running much hotter than nVidia. If you look at the new x1800 XT reviews they all say there is so much heat coming out of the card, you don't want to put your hand in front of the exhaust vent for long, as the exhaust temperature can easily reach 190 degrees fahrenheit. Because of this, I don't doubt that it's the GPU overheating. I'm not saying the Arctic Silver WILL stop your Xbox from overheating, but it will certainly help. Arctic Silver itself usually drops temps about 10 to 15 degrees fahrenheit.