Please HELP: Motorola cable box won't play nice with 27" iMac miniDP IN port!?

[Crunch]

hi all. I really hope someone can help me with this. I gave my HDTV away to a friend in need so for now, I'd like to use my new iMac as a pseudo-TV. The mini DisplayPort on the 27 inchers allows for both audio and video to flow both IN and out of the iMac, so what do I need in order to get this working? What kind of software will I need to use?

I'm connecting the cable box through its HDMI port and on the other end I have attached the (quite pricey) HDMI-to-miniDP adapter. Do I still need some kind of tuner? My cable box is brand new and also has FireWire, USB, and Ethernet ports.

How can I get this going? I did a LOT of searches, but to avail. Please help...

 

[xraydoc]

What kind of adapter, specifically, are you trying to use?
As far as I know, the 27" iMacs only supports direct 2560x1440 resolution input (i.e., from another iMac, MacBook, etc.).