Microsoft just announced a modification to configurations they will support (as from Exchange 2010 SP2).

As you might’ve read, SP2 will bring some new functionalities to Exchange 2010 such as Address Book Policies (ABP). This functionality, however, will not be available to Exchange 2010 running in hosted (multi-tenant) mode.
Because of the feedback Microsoft received, they decided to support hosting the on-prem configuration of Exchange as well. This is good news for hosted Exchange providers, as they will now be able to provide the same functionalities as with Exchange on-prem.  In order to be eligible for support, there are some requirements you’ll have to meet though.

For more information and the original announcement, please visit :

• http://blogs.technet.com/b/exchange/archive/2011/08/30/exchange-server-2010-sp2-and-support-for-hosting-exchange.aspx