XBMC (spelled like this, used to be short for Xbox Media Center as it was originally made for the first Xbox hardware) and Mythtv are NOT comparable. Mythtv is the ultimate home DVR system with all sorts of cool features plus streaming capabilities, scheduling from a web page etc. Americans might understand it best if I just say it's "tivo on steroids". XBMC on the other hand is a very nice media center software for watching videos, listening to music, playing emulators etc. Sure, both are used for watching videos, but it's like comparing Windows Media Center to Windows Media Player. Different things.

Mythtv actually consists of a frontend (program to watch recordings) and a backend (a server with tuner cards etc.). The idea would be to get a frontend to work on the N900, so you could use it as your own little television, making connections to your backend server. There used to be a basic program for N810 that actually allowed you to watch recordings and even live tv, I forget its name now, but it wasn't a full fronttend. Anyway, it was neat, I set it up once, but it somewhat buggy. The proper frontend port for the N810 never really worked properly, I think the hardware just isn't good enough for it, frankly. If the N900 were to get a fully functional Mythtv frontend, it might make my plan to wait for the Maemo 6 device a bit harder...

Mythtv is very, very cool. I've been running it for years. XBMC is very nice too, I have an old Xbox with it, use it as a media center at the family summer place. Perfect little thing for its use, watching videos and playing old emulator games.