I've seen lots of chatter / hopes / wishes / failed attempts about Exchange 2010 working with BES. Keep in mind though, that once you bring in Exchange 2010 you're changing Active Directory and limiting future possibilities ... and possibly breaking things

Some things to keep in mind about Exchange 2007 from Microsoft:

• Exchange Server 2003 is not supported in Active Directory environments whose domain/forest functional levels have been raised beyond Windows Server 2003. However,

...