A quick tip found in a couple of forums. After installing Exchange 2010 into an Exchange 2003 environment, you need to manually add the new Exchange 2010 SP1 servers to the (legacy) “Exchange Domain Servers” group. You can find the group in the Users Organizational Unit. If you don’t add the servers to this group, you will receive an Object not found error when selecting a user (which resides on Exchange 2003) for a New Local Move Request.
By adding the servers to the group, Exchange will find the Mailbox Database where the mailbox resides and you are able to move the Exchange 2003 mailbox to Exchange 2010.