I searched on this topic and didn't find anything - here goes:
I use a Mac Cube running 10.3.9 as a headless MP3/VNC server. I have my entire mp3 library on the Cube and it pipes audio into my living room - I also use the iTunes library sharing feature to access my music from my tablet pc or my pc workstation.
I know a linux client was built to allow iTunes Store purchases and downloads and playback. Don't know the status of that app right now.
I believe the following would be very useful and popular.
PSP homebrew application that uses WiFi for access to and provide playback of a shared iTunes library on the WLAN. Including the ability to save playlists on the MS would be tight.
My only contribution is that I would glady collaborate on and help create the interface art - no programming here unless you want HTML/PHP.