I found this when mucking around in mail. Here's how to create a mail account that can be checked by multiple users and have the mail downloaded into the same mailbox (so all users can read it).
Create a folder in '/' (the top level of your drive), and set the permissions to read/write for other. Then set up a new account in mail. In the advanced options, set the Account Directory to the folder you created before (do this now as this directory can't be changed after the account is created).
Then do the same process for the same account for the other user(s). Take care to fill the details exactly the same. Enter the same Account Directory and create the account. When you create an account with the same name as another account in the same directory, Mail uses the same account rather than creating another one. You can now use the account and its mailbox as a normal shared mailbox.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020913062621227