Sorry I haven't read the whole thread, so please excuse me if I am repeating anything that has already been said.
The iMacs body is very small and all the computer components are tightly packed together. The only vents for cooling are the speakers, on top of the Power plug, and at the top of the iMac for heat to rise. The iMac already builds up a lot of heat, I am running a Mid 2007 (the first Alu iMac) 2.8GHz iMac, I'm having my fans running up at about 2000RPM and my CPU at 144Fº.
Putting desktop components into the iMac will simply cause overheating. Though, the Hard drive is a desktop component, and that is the only desktop feature of the iMac besides the large 24" screen (or 20 for some). Try putting your hands on the top of your iMac after a few ours of use and you'll see what I'm talking about
