Microsoft released a first beta of Exchange Server 2010 SP1. With SP1 it is possible to create a multi-tenant hosted Exchange environment, there's even a Deployment Guide for this: http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=bba88fb5-9d84-475c-85d7-3ffeb308636e
There are a few gotchas though:
- This is targeted towards hosting companies and NOT towards Enterprise companies;
- You need a license version for this;
- You need to write your own centralized admin for this;
- There's no support for Public Folders (and therefore no Outlook 2003 support);
- There's no support for any mixed environment, so green-field only;
There will be an updated 'Address List Segregation' whitepaper for Exchange Server 2010 (http://blogs.msdn.com/dgoldman/archive/2010/05/10/critical-update-exchange-2010-address-list-segregation-and-current-support-stances.aspx). This can be used if you want to 'isolate' several departments in your Exchange environment. However, this is not an alternative for hosting customers. Microsoft is working on this whitepaper, but an ETA is unknown yet.
If you want to work on a hosted implementation of Exchange Server 2010 you have to contact your local Microsoft (sales) representative for more information.