24' iMac HDMI input solution?
Your possibilities port-wise for a 24" iMac are Ethernet, USB, and FireWire. 

I've seen interesting stuff done with converting DVI video on another computer to send over Ethernet, but not at the quality I imagine you're looking for with HDMI.

USB suffers on video. It wasn't designed to handle consistent, high levels of data that represents video.
Let's skip this.

Now FireWire: FireWire's largest users are those who work with video data, and your 24" iMac has both a FW400 and FW800 port on the back. With this in-mind I went 'google-ing' and found this:

Thomson Grass Valley ADVC-HD50

Hope this ends up being a worthy solution for you, I had difficulty finding any alternatives.

Note: I wouldn't assume this thing will handle HDCP.