In looking for the same answer (how do I recover a lost/corrupt .xml for iTunes), I came across this workaround; (Please read the ENTIRE ARTICLE BEFORE ATTEMPTING)
For the PC;
1. Back up EVERYTHING - you'll need to add some content again, so put it somewhere convenient.
2. Close iTunes
3. Copy the backup "iTunes Library.itl" file from the "Previous iTunes Libraries" directory to "My Documents\My Music\iTunes"
4. Delete the blank/corrupted "iTunes Music Library.xml" and "iTunes Library.itl" files from "My Documents\My Music\iTunes" and replace with the previously copied file
5. Open iTunes - all of your music and meta-data up to the point of the backup file will be back.
6. You'll need to add any music that you've gotten since the update. just choose "Add Music from Folder" from the iTunes menu and chose the root directory where all of your music is stored; iTunes will add all of the missing songs back to your library.
For the Mac;
1. Back up EVERYTHING - you'll need to add some content again, so put it somewhere convenient.
2. Close iTunes
3. Follow this KB article on 'opening alternate iTunes .lib files' -
http://support.apple.com/kb/HT1589
In both cases, you are essentially reverting to an older library, preferences, meta-data, etc.; Because of Time Machine, the Mac workaround is theoretical and unproven by this author, but it should work (IOW, at your own risk).
While this doesn't solve
my issue of having altered the .xml file (PC) for the purpose of syncing my iPhone with both my Mac and PC iTunes (by 'tricking' the PC-version in to thinking it was the same copy), this
may solve issues for some others.
Enjoy.