The iMacs do not accept HDMI input.
Current 27" iMac will accept video in via thunderbolt from another thunderbolt Mac.
Pre-thunderbolt the 27" would accept MDP in (I think that's the right connector)...
It's unfortunate that iMacs don't simply accept HDMI input, but there's a good reason for it - copy protection HDCP... If, for example, you wanted to connect a blu-ray player's HDMI port to the iMac then the iMac would have to be HDCP compliant, which is a huge can of smelly worms.