This hint is an update to this rather old hint which explains how to record HDTV signals from a FireWire capable box.
For the most part, everything is the same as in the original guide, however there are a few differences.
First, with the rise of FireWire 800, you should note that cable boxes supplied by your provider will have FireWire 400, so if you only have FW800 cables, you'll need to get a 400-800 cable or adaptor.
Second, Apple's Firewire SDK is rather old, and its contained applications, like VirtualDVHS, are designed for Tiger, and are rather buggy and difficult to use. A better choice is FireRecord, a simple app that does most of the work for you, including detecting which channel to stream from, which can be frustrating to do manually. (63, 0, and 1 are the common channels).
Third, while the file format (MPEG 2 transport stream) results in large files, with today's hard drives, they aren't all that large, and your startup volume will almost definitely be in HFS+, so you won't need to worry about reformatting. You can use VLC and a few other apps to playback the files, and there are a variety of apps which can convert the files.
With these differences noted, here's a step by step guide:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100830105107563