I’m not hugely knowledgeable about BES issues and even less knowledgeable about MS Exchange but I have had a tip off that the upcoming Microsoft Exchange 2007 SP2 could cause issues with your BES server. There has been issues in the past with service packs and BES so the latest release will probably cause the same type of issues as the past updates. And considering now there is 4.1.x and 5.0 versions of BES out, it will be even trickier.
If you are a BES Admin or are in charge of a BES server then suggest you check out : http://msexchangeteam.com/archive/2009/05/11/451281.aspx). It is a requirement to be able to upgrade to 2010.
For more information check out: BlackBerry Enterprise Server Resource Kit v5.0.0 Maintenance Release 2