Multiple libraries in iTunes 7という記事より。

The recently-released iTunes 7 has added the much wished-for ability to manage multiple music libraries

「iTunes 7」で複数ライブラリの利用が可能になっているそうです。切り替えの方法は簡単。

Switching libraries is simple; hold down the option key on your Mac (or shift key on Windows) when you launch iTunes et voilà, you'll get the option to create a new library or choose the library you want to load.