Move mailbox in Exchange 2003 has significant improvements related to reliability and performance. Previously, to run multiple instances of mailbox moves, you would have to open up multiple MMC consoles and initiate multiple moves, or write a script to do it and run it multiple times simultaneously. Now in Exchange 2003, we automatically use 4 threads to move mailboxes when you select multiple mailboxes to move. You can also start up multiple task wizards to run the moves. This means that you do not have to endure the overhead of multiple MMC consoles. Each task window consumes 4 thread for the moves and therefore (depending on your hardware) makes the moves faster.
There are also some reliability improvements in move mailbox in E2K3. When mailbox moves fail, you can continue after the failure without failing the entire mailbox move - you can auto-delete the mail that caused the problem. There is an XML logfile created with the results of the move, and you can create an XSLT to display the results (there's a sample on MSDN). This way, your overnight reports of how the mailbox moves went are already written you just need to format them with the xslt.