Macintouch has done excellent journalism covering the heating and reliability problems with the G5 iMac initial release:
...I have been told recently that Apple is no longer replacing the midplane assemblies, they are now replacing fans with a newly designed fan which tells me they have been having problems with their rev.1 boards. I guess the new rev. 2 boards are being reported all over the net. I can attest that the first version fans were horrible, at least the heat issue was a problem and it certainly didn't do much for quietness.The new 20" iMacs are a lot cooler than the prior model. I think they may be safe to buy.
[A new 1.8GHz Power Mac G5 is almost identical to our 20' 1.8GHz iMac G5 in architecture, but we find it's running a *lot* cooler in the big aluminum case, according to ThermographX - by some 50 degrees F. -MacInTouch]"