Tried to move to Exchange 2007, but web access doesn't work using the Exchange virtual directory. Not thinking ahead, I already started to move some mailboxes over from 2003 to 2007 before I had done a backup and such. This always leads to trouble for me. When the Exchange OWA virtual directory wouldn't work, I decided to wait to migrate, which meant I'd have to move back those mailboxes I already moved. Unfortunately, as luck would have it, moving the boss' mailbox back kept failing! His was the only one too. Couldn't understand why it was failing during the move so I was desperate trying to figure out how to move it back so his ActiveSync would work again.
Turns out having Outlook open was causing the failure. Maybe it's because he's running Vista, but closing Outlook allowed the move to succeed. I assumed Exchange would take the mailbox offline before doing the move, but I guess it doesn't.