If you've got two computers on the same subnet running iTunes, then they can share music. The server's preferences, under the "Sharing" tab, must enable the sharing. The client just magically sees the server listed as "Shared" on the left of the screen, below "Library" and "Store".

I assume, for lack of any information to the contrary, that this is the same mechanism used by AppleTV to see the music you're sharing from your computer.