I'm glad you mentioned that....
Is this (above) a new feature? Because for the longest time I though my wife's 24" iMac would have made a PERFECT 'TV' that I could watch while she wasn't using it. (or a college kid etc). The only thing I was always upset about was the fact that the flat panel iMacs only did 'video out' and their wasn't a simple way to have the iMac accept the signal from our HDMI cable boxes (yea with xtra hardware I could but I said simple (and cheap/free).
This is really great news cause I can get 1 HD cable box have it route to a HDMI switcher (remote operated) and have mine and my wife's (future) iMacs both connected to the HDMI switch.
This way whoever isn't using their system can do double duty as the computer room TV.