You can access your shared iTunes playlists through a protocol named "daap" (Digital Audio Access Protocol) on port 3689/tcp.
daap://host.domain.name and hit return, and iTunes 4 will launch and attempt to connect to the remote server. A nice way to bookmark your music servers.
Another interesting protocol is
itms that links directly to Apple's iTunes Music Store search function.
Where search term can be any term below in any order:
songTerm - song title
artistTerm - artist name
albumTerm - album name
composerTerm - composer name
An Apple KnowledgeBase Article confirms that you need to save your purchased tracks to an external media (sic).
Be sure to make regular backups of your music files (in your iTunes Music folder) by copying them to an external hard disk or other media. If your hard disk becomes damaged, or you lose any of the music you've purchased, you'll have to reimport all your songs and buy any purchased music again to rebuild your library. You can also make an audio CD of the songs you purchase so you can listen to them in a consumer CD player.The copyright notice at the end of the article is well worth a smile,
If you are uncertain about your right to copy any material, you should contact your legal advisor.