Uhm, Exchange 2007 (still in Beta) should not be considered to operate in a production environment until its release. So I don't see why anyone would want to bother install BES on it at this stage as the last thing I would want is to have sleepless nights over getting it to properly work. Plus keep in mind what it takes you to get it working now may not require you do the same on the production release of Exchange 2007.

Andrew