http://docs.info.apple.com/article.html?artnum=60920"Question: I have a computer at home and one at work. Can I update my iPod music collection from more than one computer?
Answer: Yes. When you first connect iPod to your computer, iPod recognizes that computer as its "home" computer. Each time you connect, iPod automatically downloads the music library stored on it. When you connect iPod to a different computer, iPod asks for permission before synchronizing with this music library. If you choose Auto-sync, iPod recognizes the second computer as its home computer and breaks the link with the original computer. If you do not choose Auto-sync, iPod switches to manual update mode, allowing you to selectively drag songs, albums, or playlists from the second computer to your iPod.
Synchronization occurs only in one direction, from your computer to your iPod. This means you cannot transfer music, automatically or manually, from your iPod to a computer, and
you cannot use iPod to copy a music library from one computer to another. But, if you legally are allowed to copy music files, you can configure your iPod as a hard disk to move music files."