I'm wondering the same thing. No one has done a run through of how using the display port as input works, yet.
Quote from reynhout:
Re: Mini DisplayPort, you write:
> you'll need to have your iMac powered on if you want to
> display from an external video source.
This means "powered on and running OSX", right?
So, this iMac won't become a decent second display if the logic board goes south, or after quad 2.66GHzs are no longer "fast enough" to read email on?