I am having the same problems with itunes on xp. I also have mt-daapd on ubuntu 8.04 (hardy).
If i open itunes the music share is not there.
But when i leave itunes open and give mt-daapd a restart the share becomes available.
So on ubuntu 8.04 mt-daapd only makes itself known when it is starten and not later on. So the part that makes it known to the rest of the network stops working.
Also i have to disable my nod32 firewall. So you may also want to look at that. Dont know if osx has an firewall.