Something I had not known until reading about it in the X4U mailing list: apparently, Time Machine does not back up the "iTunes Music Library.xml" file. This is the file that is periodically updated by iTunes which contains track and playlist data. It isn't used by iTunes, but it can be used to re-import data and what not. Most of the time, this file is located in your [username]/Music/iTunes/ folder (you should not relocate it). Thus, if you are concerned about this file, you may have to manually back it up, independent of Time Machine.
UPDATE: A post at Mac OS X Hints discusses what files are ignored by Time Machine and how to list them.
UPDATE ALSO: Any copy of the "iTunes Music Library.xml" file that is not [username]/Music/iTunes/Tunes Music Library.xml will get backed up. So periodically copying it as something like [username]/Music/iTunes/bu/Tunes Music Library.xml will ensure Time Machine sees it.