A nice trend I have been seeing in the industry is the addition of Tivo functionality to your standard run-o-the mill computer.
I was nosing around and found this:
http://www.ajump.com/ajump/media_center.asp
Which is based on the nice XPC size cases coming out so it fits right into your media center without any problems. I have a friend who is going to be building one of these based on the Microsoft MediaCenter OS. I will keep you all posted on the success of this since I am planning on adding one of these to my setup as well, if anyone is intrested that is.