Yes, it can be done! I've almost finished building a digital pvr (personal video recorder). The only thing left is getting the HD tuner connected to an antenna to record OTA HD. The price-point is higher than Tivo, but you gain a LOT of flexibility like adding as much disk as you can afford, multiple tuners (SD and HD), a web interface to schedule and view programming, burn DVD's of recorded shows, automatically skip commercials, etc. etc. I have mine sitting in the basement running 'headless' and watch stuff on the (modded) XBox. Very slick! You can have multiple clients all feeding from one backend server, so if you needed a PVR for you and your roommates, you could put a big box with 3-4 tuners and network it to multiple xboxes or linux clients or windows clients!
(See the links in the extended entry for details)
Links to MythTV resources:
http://www.mythtv.org - The base project
http://www.mysettopbox.tv - where to get KnoppMyth R5A22 to install from. It makes building a PVR nearly trivial
http://labs.zap2it.com - Where to sign up for free listings, you need to fill out a 3-4 question survey every three months to keep the service, but it's basically (How many shows a month do you record? What PVR software do you use? etc. It takes like 60 seconds to complete)
http://winmyth.sourceforge.net/ - The Windows front end
http://sourceforge.net/projects/xbmcmythtv/ - The XBox+XBMC Front end
http://bit.blkbk.com/ - The XBox+Linux Front end (I haven't tried this yet, but I would recommend using xbmcmythtv above...
http://www.ntbrad.com/ - My site with the whole (kinda tedious) story, but details are there for building one yourself.
Go project Mayhem!!! thanks for the info! must try it ! i have a high def box , xbox and lots of experience configuring PM!! you rok! will kepp ya posted