Are there any solutions available which will allow HDMI input on my 24' iMac?
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:
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.
No, there aren't. The 24" iMacs don't take video input of any kind. If you had a 27" iMac, then it would take Mini DisplayPort input, and you might be able to find an HDMI to Mini DisplayPort converter.