There is at least one tool listed at http://www.slipstick.com/addins/content_control.htm that can pull that information from AD and apply it as the message goes through the server, so you don't have to worry about the users editing or deleting the signature on their own.
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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers