Actually it depends, the 27" iMacs (until the latest range) have a mini-displayport port so if you can find the adaptor and have such an iMac, you can use it as an input
I dug this thread up on Google. I have a 21 inch iMac purchased last fall. I'm considering buying a Mac mini and using the iMac as an external monitor, but so far it sounds like I can only do that if i have a 27 inch monitor. is this true? thanks - aldri7