There are around a half a dozen people working on the Mac Media Center project. It’s an open source project that is looking to develop software that will allow your mac to act like a TiVo, allowing you to record and play back television amongst other things. If you’re a bleeding-edge Mac user, you’re welcome to download the Pre-Alpha build of the software. The rest of us should wait and keep watch on this site to keep up with future developments.
I’ve started a new mailing list dedicated to using the Apple Mac or Mac Mini as a Media Centre. If you’re interested, join up!