Here is a shameless copy and paste from a deck by Microsoft Communication Sector Industry Director. A pretty good high level diagram that tells you 2 different scenarios and how you can move to Exchange 2010 Hosting Deployment. Here, a picture says it all,
For both scenarios, you need to manually migrate the users as well as the mailboxes to the new forest. Of course, there will be some specific guidance as to where to move the objects to and that you need to understand the new Organization structure in AD in Exchange 2010 SP1 Hosting Deployment.
Hosting Scenario #1
If you are currently using a 3rd party control panel which leverage the HMC/MPS framework, then this scenario is for you. The migration is probably going to be pretty straightforward, as in your control panel vendor provider will probably have some migration guidance for you to do that.
Hosting Scenario #2
If you are currently using your very own customized control panel, then you should consider either to buy a 3rd party control panel and if you are one of those brave souls or you really very specific things that no other 3rd party control panel can provide, then you need to look at how to convert your control panel to cater for the new Exchange 2010 SP1 Hosting Deployment model. There should be a migration guidance document come up from Microsoft soon to help you on this. ETA, don't know yet.