If you've encouraged to upgrade to iOS 5 [- and therefore to iTunes 10.5 beta -] and then you wanted to downgrade, chances are you've encountered error iTunes does not allow you to upload your iTunes library have been created with a completely new version.
The solution is simple:
Go to ~ / Music / iTunes or where you have located your iTunes library.
Rename the file "iTunes Library.itl" to "iTunes Library.itl.old".
Go to the "Previous iTunes Libraries" folder and find the newest file.
Rename this file to "iTunes Library.itl" and put it in your iTunes folder
ITunes starts normally.
The solution is not complicated, and within a minute should be fixed without major problems.