So I've been doing some more reading into my Digital home theatre project (it seems I've been doing a lot of reading lately). I came across an alternative to MythTV: Freevo. As the name suggests, this is intended to be a free (as in you put hours of work into something and place no actual monetary value on your time) alternative to TiVo.
It looks interesting. I'm not sure if there is any general consensus as to which is better, but it is probably worth some additional investigation.
I did find a bunch of comments on someone's digg post that argue both sides of the fence, which wasn't very helpful. I wanted to post a screenshot, but the ones on Freevo's site don't seem to work (which isn't very encouraging).