@PastAmigaOwner: All the Macs I've taken apart have HUGE heatsinks, though. The G3 I used to have at work has a heatsink about 18 square inches, with a 120mm case fan blowing on it. That's one advantage of being able to build custom cases. The G5 Mac also has a case fan in front of each CPU heatsink. That's hardly passive cooling. Maybe Apple is really pushing these processors to their limit?
I also heard the Cube had overheating problems, because the video chip gets too hot and thus affects the CPU. It doesn't matter to the end user what the problem is.