Wirelessly posted (BlackBerry 9000)
I don't think this is by design; a couple of years ago I used BIS with Exchange 2003 before I got my BES, and I did not have this issue. I don't know what's causing it, but I don't think the issue exists with every Exchange server using BIS. It should continue to work fine after the server reboots.
BES 4.1.6 Exchange 2003 SP2