After lots of toil, I finally was able to get multiple users to share one set of mailboxes in Mail.app. This is great if you have multiple users who are sharing one e-mail account, as it keeps each user from having to delete every single junk e-mail.
To start off with, you should probably backup each user's ~/Library -> Preferences -> com.apple.mail.plist file. You also need to have one user whose box setup will provide the model for the rest of the users. Once you've decided who's going to provide the "model box set," copy their ~/Library -> Mail directory to /Users -> Shared.
Now you need edit each user's ~/Library -> Preferences -> com.apple.mail.plist file so that Mail.app will be directed to the new location. I used TextEdit while logged in as root, and used Find/Change to change all of the ~/Library/ entries to /Users/Shared. The last thing that you need to do is change the permissions on the Mail directory so that all users can modify it. In the Terminal, type sudo chmod -R 777 /Users/Shared/Mail, and enter your admin password.
At this point, you can have the users test out the "new" Mail.app. If you don't like the results, just go back to your ~/Library -> Preferences -> com.apple.mail.plist file, and change all of the /Users/Shared entries back to ~/Library. Then you'll be back to the old fashioned Mail.app as before the changes were made.
[Editor's note: There are probably some security concerns to consider with the Mail directory being world readable and writable. If this concerns you, you could create a new group in NetInfo Manager to allow access ony by specific people, though I haven't tried that myself. You may also want to back up the Mail directories before you try this, especially for the person who will serve as the "model" mailbox for all to use.]
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