iTunes on the PC on my desk (notice I feel more possessive of the desk than the PC) hasn’t been able to share music to or from iTunes on my PowerBook. Blame the firewall.
Port 3689 TCP
Port 5353 UDP
However, he notes:
Mac OS X users have it slightly simpler. They only need to allow one port through [port 3689 TCP], for reasons that are unclear to me as yet (I’m guessing the Mac firewall is at least partially stateful, which would explain this).