there were unsubstantiated rumours a couple of months ago about a german guy who got a late model ibook to use a second monitor. he posted pics but of course they could have been faked.
what happened was that he copied the contents of his powerbook which had the same video card over to his ibook and when he rebooted in OS9 it seemed to support two monitors. unfortunately he had no luck with X. the idea was that the drivers were the key.
here it is
there may be similar tricks to be had with an imac