I use this hint to share both iTunes and iPhoto directories among users on a local machine. This works very well, however one difficulty arises which relates to fast user switching. If someone has iPhoto or iTunes open, other users are prevented from running or modifying the database (you get a "database locked" error). To solve this, install WinSwitch, a replacement for Apple's FUS tool with many improved features (including a lovely small icon view for the menu bar). After installation, open the WinSwitch preferences, and click on "Go to Switch-Out Items". This directory contains scripts and aliases which will be run when you switch to another user.
Open the Script Editor, and create a script with these two lines:
tell application "iPhoto" to quit
tell application "iTunes" to quit
Save this as an application (I called it Quit iTunes and iPhoto, and stored it in the Applications folder). Command-option drag this app to the Switch-Out Items folder to create an alias. For each user who shares iTunes and/or iPhoto, install WinSwitch and create a similar alias.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061212090013611