sudo aptitude install mt-daapd
mp3_dir = [your music directory]. (Also any other configuration changes you might want to make; the file is well-commented.)
sudo /etc/init.d/mt-daapd restart
Really this would work on any Debian-derived distro… but I’ve only tried it on Ubuntu.
Update: It looks like mt-daapd puts an admin interface on http://localhost:3689/ (with the default password being ‘mt-daapd’), which you can use to configure things like the location of your music. So you might not even need to do any config file editing…