I'm not familiar with what video cards Macintosh is using, but does it have a DVI in? Almost all modern video cards have 1 or two DVI in..
If so, how about an HDMI to DVI cable or adapter?
Newegg.com - Link Depot 10 ft. HDMI To DVI Cable, Black Model HDMI-DVI-3 - Cables
Cheaper on eBay, too, i bet.
EDIT: I'd also take a really close look at the back of your cable/satellite box. Most boxes new enough to have HDMI out also have 1394 out, and you could just do firewire to firewire..
EDIT x2: A tidbit I found about Mac with firewire connection to cable box:
"If your cable box has the firewire port, then all you need is a firewire cable, a Mac with firewire input, and the magic D-VCR software from Apples development site (free, you have to register though). I think the applications are part of the Firewire SDK kit in the download section of the Apple Developers site. Once you download the Firewire SDK, you will find three applications AVCBrowser, DVHSCap and AVCVideoCap."
EDIT x3: Walkthrough here:
http://www.avsforum.com/avs-vb/showthread.php?t=386740