The Exchange Team blog has a great post on the differences in mailbox moves between Exchange 2007 and 2010 here http://msexchangeteam.com/archive/2010/07/19/455550.aspx. It covers some of the services used (MRS) and how CAS plays its role, plus a load of other stuff.
I really like that from Exchange 2007 we can do online moves which only require <30 second outage to the end users. This means that we can move out entire Hosted Exchange 2007 platform to 2010 during the day, without causing downtime to our customers.
I’m quite a fan of the resiliency built in to this now and that you no longer have to leave a PowerShell window or GUI open. An exciting addition is the simplicity of cross-forest moves, this makes the task on on-boarding/off-boarding for our customers so much easier.
I’m not going to repeat the blog post so head here http://msexchangeteam.com/archive/2010/07/19/455550.aspx to read it.
Daniel