After struggling with an option that allowed me to story my main iTunes library on a file server, whilst still having a second (smaller) library that I can take with me when I'm on the road, I concocted the following concept. Create a second user account specifically for sharing music and relocate the iTunes library to the external server. Then share the music.
Go back to your main account, and you'll have access to this new library through iTunes wonderful sharing ability.
This has a few advantages over the option of creating a Rendezvous-capable linux box: Music can still be ripped to the main library, and playlists can still be created and edited; all with a simple fast user switch.
[robg adds: My question to the author was "why do you need a second user? Why not just use the external server from your primary user?" His response was that he wanted a smaller library stored directly on his machine (a PowerBook). Now, when he travels, the larger 'shared' library just vanishes, and he's left with the smaller local library, all with no copying required.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040620010210953