I too had a similar issue:
BES 4.0 on 2003 Server
MS Exchange 2003
All rights where given to besadmin user on the exchange mailbox for the stuck user
User stuck on the BES at initializing
8800 Blackberry Enterprise Activation on the handheld would not activate
Checked my anti-spam email software on the Exchange server and noticed that an attachment with a .DAT extension was being sent to the user from
when the Enterprise Activiation software on the handheld was trying to activate and was being blocked.
Removed .DAT from the blocked extension list in my anti-spam email software on the Exchange server and the handheld activated without issue.