However lets do a quick recap...
Transport Dumpster resides on all Hub Transport roles on both Exchange 2007/2010. All messages which get delivered to users mailboxes is routed through a hub transport server and stored in Transport Dumpster. In the event an email is sent from one user to another user on the same mailbox server, the mailbox server routes the email to a hub transport server and back again through MAPI. This is to ensure that things such as journaling rules, transport rules and any other transport agents take effect, the message is filtered for malware/viruses (if configured), the message is trackable using message tracking logs and the message is copied in transport dumpster for a small period of time.
The following diagram shows what happens when Joe sent an email to Bob on the same mailbox server, the message goes to a hub transport server in the same AD site then back again to the mailbox server.