Just to give it a bit more google juice: ushare can act as a replacement for Windows Media Center for playing media hosted on a linux box on your XBox 360. I googled for quite a while before figuring out what exactly I needed (and I learned that UPNP has media protocols. wacky.)
1. Install the Ubuntu .deb (you're using Ubuntu, right?). The direct link on their web site to the .deb is a 404 -- I browsed their pool directory to find the directory with ushare debs. Download whichever one you need, for your 32bit or 64bit OS. You can also add their deb line to your Software Sources.
2. Edit the configuration file to change which interface to listen on, because the -i (--interface) option doesn't actually work (or the configuration file takes precedence). Pretty lame.
3. Run "ushare --xbox --content ~/Media".
4. Go to the media tab of your Xbox 360 and browse movies and music. They'll be listed.
There are several problems. First, I haven't managed to play any movies — I probably just need to transcode them to whatever codec the 360 supports with mencoder. Second, all unrecognized files seem to end up under the Music section, so there's lots of cruft in there. Third, it doesn't seem to recognize Artists, Albums, Playlists or Genres separately from Songs. That is, all of my files are showing up in all of those sections through the 360 media browser. I guess that this is a ushare bug, but I don't know. The last problem is that it seems music will just stop playing in the middle of a song. I tried starting the music again but it just froze my entire xbox. It looks like ushare still needs some tweaks to be less flaky.
Oh yeah, btw. I got an Xbox 360. Sorry guys. Halo 3 is really awesome. I'm radeex in the Recreation zone. Friend me.