This can be done by installing Exchange 2010 with SP1 (this is in Beta right now)
you need to install from the command line with the /hosting switch
you can download the public beta here
http://www.microsoft.com/downloads/details.aspx?FamilyID=a0965fc9-2723-4947-ae6b-74bc3808e72a&displaylang=enthe release is not expected for a few months yet so it is not supported for a production environment yet.
Exchange 2010 SP1 doesn't support the following features in Hosting mode:
•Exchange Management Console
•Public Folders
•Unified Messaging Server role
•Federation
•Business-to-Business features such as cross-premises message tracking and calendar sharing
•IRM
•Outlook 2003 support (EnableLegacyOutlook)
•Edge Transport Server role
Here is a good set of articles on setting it up and using it...
http://blogs.technet.com/b/provtest/archive/2010/06/25/exchange-server-2010-sp1-beta-hosting-deployment-the-first-look-part-1.aspx