Does your phone have enough signal? you need between 40-99 to complete activation successfully - to see your signal strength from the main window type the following: alt +nmll, Have you tried a hard reset(removing battery) and reactivating?
Does it stop, or simply slow down...you can often see it "stuck" on a certain %age for a while as it's syncronising a large portion of the mailbox e.g. Calendar/Contacts.
BlackBerry Enterprise Activation
