What's the best way to speed up mailbox moves between Exchange Server sites?

Reduce the amount of data you have to move. When you can get users to clean up their mailboxes before you move them, you'll benefit in direct proportion to the reduction in data size. If you have a long lead time before the move, try using Mailbox Manager to archive older items. If at all possible, use the Exchange Server 2003 version of Exchange System Manager (ESM) to move mailboxes, even if you're moving those mailboxes between two Exchange 2000 Server machines. The Exchange 2000 version of ESM handles all mailbox moves through the use of a single thread, which acts as a rather effective bottleneck. In Exchange 2003, however, ESM can use multiple threads to move multiple mailboxes in parallel.