So, have you tested Exchange 2010 SP1 beta yet? If not, you should really start looking into it if you like to maintain competitiveness by providing the latest and the greatest to your users in this hosting world. The changes are quite huge. As most of you know, HMC 4.5 is the last version of this Hosted Solution as in the following link,
"With the release of Microsoft Exchange Server 2010, Microsoft is creating a new way to support hosted Exchange deployments. Hosting in Exchange Server 2010 will be available approximately twelve months after the release of Exchange Server 2010. HMC 4.5 is the last version of this solution. Microsoft will support HMC 4.5 and HMC 4.0 through Dec 2011. HMC 3.5 support has ended."
To put it simply, Exchange 2010 and SharePoint 2010 for instance the products will be natively mutli-tenant and will be tested by the product group instead of a separate HMC solutions team. In the new model, Microsoft is taking a slightly different approach compared to HMC. In HMC, Microsoft provides almost the full solution (except for the Control Panel) and detail steps on how to setup the whole hosted environment. It includes resource management, service plan management, multiple services orchestration (SharePoint, OCS, CRM, Exchange and etc.), roll back and transaction orchestration and etc.
As we move towards Exchange 2010 SP1, you will see some of the HMC functionalities will be moved to become native features of Exchange, for example, building and provisioning the multi-tenant architecture in AD, provisioning tenants, and users, managing and isolating address books and DLs, etc. However, some features for instance will move up the stack to be built into the control panel tier, like cross product provisioning, or resource management. To put it in the HMC terms, the Providers layer and some of the Managed Layer will be built into the Exchange 2010 SP1 product itself. Hosted layer and those logic, Microsoft will leave that pretty open for you guys to design and implement.
The rationale behind of this approach and removing this HMC middle layer is reducing the complexity of deployment , management, and overhead, and achieving higher scale and wider support of features that we haven't considered before.
Here is a quick table summarizing the stuff. Note the highlighted portions are the portions which used to be covered by MPS and now you will need to get a 3rd party control panel to do the job or you have to design those logic yourself.
Wait, wait, wait... you may ask... is there any migration guide from HMC to this new Exchange 2010 SP1? As far as I know, yes and it will come much later and with no official ETA yet. However, the migration guide will probably be just for HMC 4.5 only. If you have HMC 3.5... you may be out of luck on this one. Perhaps it is time to upgrade.
I will walk through the Hosting deployment with you guys over here too next. Hang on there.
Please take note that this information is provided as it is and it is by no mean be considered as official information from Microsoft. You should always contact Microsoft and refer to the Microsoft help file to confirm.