The VAR Guy has a post about the dilemma of Hosted Exchange 2010 - on premise, in your data center on a dedicated box per customer, or fully hosted. He also mentions OpenXchange, the open source version of exchange. What to do? There is are other choices like Zimbra (owned by Yahoo but open source as well); HyperOffice; Google Apps; and ZoHo. If you need Blackberry Server, there is Funambol (open source version).
So go make some money on hosted email.