Some people have set up iTunes to keep the library files and other resources in a shared location so they can access them from multiple accounts. When updating iTunes; however, you may lose this functionality.
Apple discussion poster "bk1000" writes:
"I had my iTunes library in my shared user folder, and I could share the library between the two accounts on my iMac (one account for me, the other for my wife). Now I can still access it, but my wife can't. iTunes reports the file is locked, even though iTunes ISN'T running on my side."
If you have set up iTunes files to be accessible by multiple people in this way, remember that updating iTunes will have the program update the library and move the old one to the "Previous iTunes Libraries" folder. This will reset permissions on the file, and potentially change how others can access it.
In order to fix the problem this user got information on the top-level shared folder containing the iTunes files and resources, and used the "apply to all enclosed items" option in the gear menu to ensure the files in the folder were accessible as they previously were.
If you are interested in sharing your iTunes folder with other computers in this manner, you might be interested in checking out the new "Home Sharing" feature in iTunes 9.