Here is what I followed. This was posted on the Apple support boards.
This brought me back to iTunes 10.5.3 without any problems.
1. Quit iTunes (if it hasn't already quit itself)
2. Go to /Applications/ and click iTunes.app
3. Press command + I to get info
4. Edit the permissions so everyone has read/write access
5. Move iTunes 10.6 to the trash
6. Go to ~/Music/iTunes (or wherever your iTunes library is stored)
7. Copy iTunes Library.itl to your desktop to be safe (delete it when you
finish the process, this is your 10.6 library which you may still want if
you can't fix iTunes after this)
8. Go to ~/Music/iTunes/Previous iTunes Libraries/ and copy the file that
was created closest to today (in my case, it was one from about an
hour before I installed 10.6); the old library you're copying is your
iTunes 10.5.3 library
9. Paste it in ~/Music/iTunes
10. Rename the file to iTunes Library.itl
11. Go to ~/Library/Preferences and delete everything that starts with
com.apple.iTunes
12. Download the iTunes 10.5.3 disk image from
http://support.apple.com/kb/DL1426 find the link near the bottom of the page for:
Download iTunes 10.5.3 for OS X here: iTunes 10.5.3 for OS X
13. Install iTunes 10.5.3; it should load the older library you just copied to
your iTunes folder
14. Open iTunes
15. Play a victory song and wait for 10.6.1 so you don't have to do this again