We discovered and made a very important change to our Migration Guide.
When it comes time to move the mailboxes in Phase 4 Step 6 we need to verify the size of the largest mailbox on the source server.
Exchange 2010 is set to 2GB out of the box so any mailbox move command run on a mailbox that is over that size will fail.
So, we change the Standard User Role and the actual defaults in Exchange 2010 to accommodate the largest mailbox.
Microsoft Small Business Specialists
Co-Author: SBS 2008 Blueprint Book