HDTV + Firewire
Record HDTV with Firewire
MacOSXHints has a succinct tutorial on how to use a standard cable box with IEEE 1394 output (you can get one - free)and VirtualDVHS to record HDTV signals. These files are huge, clocking in at more than a dozen gigs for a feature length film.
You can play back the video with VLC.
The firewire connection will also allow you to send video from your Mac to the cable box.