Well, there are three separate functions being talked about here. I don't think you can do all of them with a single port.
1) HDMI input which would go directly to the display. This would let you use the iMac like a TV. Currently this only works using a Thunderbolt cable and only with other Thunderbolt equipped computers.
2) HDMI video capture port which would let you record the HDMI input from within OSX. This can be done currently but requires an external video capture box.
3) HDMI output, which would let you hook the iMac up to a TV. This you can already do with a Mini-Displayport to HDMI cable.