I had a problem that looked like this — recently updated my Ubuntu “Hardy” machine, and iTunes on my mac mini wasn’t seeing the firefly share any longer.
What I did was go onto the Ubuntu machine, check to see if avahi-daemon was running:
$ sudo /etc/init.d/avahi-daemon status
Saw that it wasn’t so started it:
$ sudo /etc/init.d/avahi-daemon start
Then restarted mt-daapd:
$ sudo /etc/init.d/mt-daapd restart
After doing this, the share showed up in iTunes.