In Exchange 2007, one can schedule the move of one or more mailboxes for after-hours times.
In Exchange 2010, one can move mailboxes while users are using them (!), without the users knowing anything is happening.
Well done guys!
In the Exchange GUI, just go under Server Configuration and Mailbox, and create a new mailbox, and then go to Recipient Configuration and Mailbox, choose some to move, right-click, and start the process.