Mac OS X
/Users/username/Music/iTunes/iTunes 4 Music Library
/Users/username/Music/iTunes/iTunes Music Library.xml
\Documents and Settings\username\My Documents\My Music\iTunes\iTunes 4 Music Library.itl
\Documents and Settings\username\My Documents\My Music\iTunes\iTunes Music Library.xml
iTunes 4 Music Library (iTunes 4 Music Library.itl in Windows)
This file is a database of the songs in your library and the playlists you've created. If you delete the file, iTunes creates a new, empty copy when you open the application, but any playlists, song ratings, comments, or other information you created is lost. The iTunes 4 Music Library file is only used by iTunes.
Tip: Older versions of iTunes you may have installed and used on the computer created their own version of the file, such as: 'iTunes 3 Music Library' or 'iTunes Music Library (2)'. When you installed and open a new version of iTunes, information from these older files is imported into the new database file. You can delete the older iTunes Library files.
iTunes Music Library.xml
This file contains some (but not all) of the same information stored in the iTunes 4 Music Library file. The purpose of the iTunes Music Library.xml file is to make your music available to other applications on your computer. In Mac OS X other iLife applications (like iPhoto, iDVD, and iMovie) use this file to make it easier for you to add music from your iTunes library to your projects.
So what happens if I delete this one? I need to look at it and try!