iFixIt has torn up the 20-inch iMac, finding inside what appears to be a computer manufactured by Apple Computer. The coolest thing? The front screen is held in by magnets and can be removed with suction cups.
The iMacs also have laptop optical drives but full-sized desktop hard drives, a fairly interesting design choice. Sticking all of those electronics into a package that small must be pretty tough. Also, you can upgrade the RAM by removing a screw in the back. Easy peasy.