To all those worrying about the heat of the iMac case, it is better to have the CPU running at 65C with the case at (say) 55C than to have the CPU at 95C and the case at 25C. As has been mentioned already, the iMac case is used as a secondary heatsink which is a good thing! Really it is!
Think of your car engine, would you prefer the engine to be at 100C and the radiator at 80C or would you prefer the engine to be at 120C and the radiator at 60C? Both scenarios have the same average temperature (90C) but one will cause the engine to overheat.
As an engineer (electronics) I can say that I have absolutely no concerns about the thermal design of my iMac. In fact it is way better than any tower based PC I have ever owned and is highly energy efficient and ultra-quiet at the same time. A really great piece of design from Apple IMHO.
Users should concentrate on "using" and let the engineers engineer!
No disrespect intended here by the way - I am a good engineer but a crap graphics designer. I'm sure there are many graphics designers out there who don't know anything about engineering (for example).