How to install BlackBerry Enterprise Server Express on a SBS 2008 is a great technical article (and also a downloadable document) written by Mariette Knap, with the help from Jason Miller, Les Connor and Michael B. Smith.
In Michael words:
“I believe that this posting is the best resource for installing BES/X available right now; whether you are installing BES/X on an Exchange Server, on a Domain Controller/Group Catalog server, or on a standalone server.
If you are installing BES/X on a DC/GC or directly on an Exchange server, I highly recommend you follow the instructions contained in that blog post”