In previous versions of Exchange, the move mailbox process was a pretty “dumb” process. You selected the mailboxes you wanted moved and then moved them either immediately or at a later time you selected. You had to leave the console running for the process to run.
Exchange 2010 / SBS 2011 Standard changes all that now. With the moving of mailboxes, when you select the mailboxes to move, you issue a Local Move Request for the selected mailboxes. This then puts those mailboxes into a Move Request Queue which is a serviced by the Microsoft Exchange Mailbox Replication service. This service then processes the mailboxes in the order that they were placed in the queue moving them from server to server.
The cool thing about this is that you can grab a group of mailboxes in whatever order you desire to move them in a specific order. For example, on a recent migration, I used the Exchange 2003 console to identify which mailboxes were below 500MB, and then I used the Exchange 2010 console to multi select those and process a Local Move Request against those mailboxes. This started the move on those smaller mailboxes and I was able to then select all the larger mailboxes and add them to the Move Request queue.
I love it when Microsoft make cool improvements like this – makes our life as IT Professionals easier.