I have been working on figuring out how to move our 20,000 mailboxes to an Exchange 2007 environment. The following script is one of the first steps. This script goes through each mail-enabled user account and tags one of the extensionAttributes with a number 1-number of mail stores I will end up with.

Once this is done, I can then use the Exchange Management Shell to move the mailboxes. By issuing a get-mailbox -Filter {CustomAttribute2 -eq 'tag'}|move-mailbox -TargetDatbase EnterYourTarget

Anyhow, please feel free to look through the code and comment or use. Cheers!