Quote from arbath:

I always wondered if you could utilize just the monitor of an iMac once the machine becomes obsolete. Maybe there is a way to connect to the LCD and plug it into a Mac mini?

You can kinda-sorta do that if the computer still works using the prefpane teleport - it connects via VNC and relays all mousing and keypresses to the other computer. Save for network lag and and the fact that you cannot really move windows between the screen it works like a computer in extended desktop mode. Of course, it also means that the old computer has to run your apps - which has the benefit on the other hand that your main machine won't have to.