Some notes regarding iTunes 4:
The existing Winamp plugins for AAC/MP4 files (FAAD) don't play the preview streams from Apple's Music Store, or AAC files encoded with iTunes. Quicktime 6.2 for Windows, when it arrives, probably will.
18.104.22.168/storeBag.xml.gz is the starting point for the Music Store, but the browseStore part seems to be encrypted.
This is why you need to password-protect your shared library.