This concept is not new in Exchange 2003 and actualy has been around since Exchange 5.5. It is a faily powerfull feature is you bare in mind that a lot of emails these days carry large attachments.
Microsoft Exchange Server 5.5, Microsoft Exchange 2000 Server, and Microsoft Exchange Server 2003 maintain single-instance storage of messages. If a message is sent to one recipient and it is copied to 20 other recipients who reside in the same mailbox store, Exchange maintains only one copy of the message in its database. Pointers are then created. These pointers link both the original recipient and the 20 additional recipients to the original message. If the original recipient and the 20 additional recipients are moved to another mailbox store, only one copy of the message is maintained in the new mailbox store.
Update 5-12-2005 : the missing link : http://support.microsoft.com/default.aspx?scid=kb;en-us;175481