Yes and know. It's easy to hook up the iMac to an external. You lose the internal in the process, but getting it back is a matter of plugging it back in. My rev A's screen died, so I hooked it up to a sony 17", works great. I guess if it hadn't died you could've hooked up anothe computer to the internal moniter, but you would need to power up the machine to run the moniter. Also you would need a adaptor, as the iMac screen uses the OLD mac moniter connector. (The one used on the Beige G3, all the way back to the Mac II.
Pointless really. Buy a nice 17" moniter cheap.