I have a G4 iMac and a G4 Aluminum PowerBook. I use a Linksys router/wireless port as my on-ramp for the PowerBook when I want to browse the internet from the comfort of my living room. All my music is on an external drive attached to the desktop machine (which has powered speakers), and so I was very intrigued by the AirPort Express as I want to listen to music from my stereo in the living room. Instructions I found in the Ars Technica AirPort Express Review (on the Bridging page) enabled me to configure the AirPort Express as a bridge to the Linksys router, and so allowed me to play music from either computer to my stereo.
One important step is to use the AirPort Admin utility from the PowerBook to configure the AirPort; I was not able to see it from the iMac until this was done.
The streaming of music to the AirPort is user specific. This means that I could login to my computer as one user, run iTunes and direct the output to my local speakers. I could then do a quick 'switch users' (not logout, so my music continues to play) and then login as another user. As the second user, I am able to run another instance of iTunes and direct that output to the AirPort. Voila -- one music source and two outputs, each with their own playlists! Brilliant!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040724234358161