Well, actually it's about “MythTV, Ubuntu, Athlon64, WinTVPVR, Comcast Digital Cable, and a pcHDTV HD-3000 in a pear tree!”
Read the massive tutorial here.
I've prepared this document in an attempt to provide a comprehensive set of instructions on how to get MythTV running on an Athlon64 with Ubuntu, a WinTV PVR-250, pcHDTV HD-3000, and firewire connection to a Motorola DCT-6200 cable box. This is not going to be a 100% complete walkthrough beacuse some things, like setting up the database, didn't have to be done again. Yes, this was not going to be an easy task.