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.
A moment of Googline led me to Travis Saling’s guide to enabling iTunes sharing through a firewall. Here’s the ports that need to be open:
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).