I wrote a nifty little Python module (for OS X only) that allows you to control and record FireWire-connected cable boxes such as the new ones from Comcast! It's simple code, and this post contains a ton of information about how it works and how to use it.
[robg adds: I don't have cable, so I can't test this one ... in glancing at the page, though, it's clearly a CLI-only tool at present, though Ken states he has a close-to-ready GUI to use with it. This older hint provides another option for recording from FireWire cable boxes.]
Mac OS X Hints