Exchange 2007 SP1 introduces several new features for the Mailbox server role. One od them that is closely related to Hosted Messaging & Collaboration is the ability to import and export mailbox by using .pst files.

Exchange Server 2007 SP1 introduces a new cmdlet Export-Mailbox to export mailbox data to either a folder or a .pst files. Messages that are exported can be filtered based on recipients and senders. For more information about how to filter based on recipients and senders, see How to Export Mailbox Data.

Cmdlet Export-Mailbox together with Move-Mailbox and Restore Mailbox, offer complete replacement for the Microsoft Exchange Server Mailbox Merge Wizard (ExMerge.exe) to export mailbox data. For more information about how to export mailbox data, scenarios by using ExMerge.exe in Exchange 2003 or Exchange 2000, and the solution for performing this task in Exchange 2007, see How to Export Mailbox Data.

The exported or imported PST file can be larger than 2GB. 2GB is the limitation of ExMerge.
There is no Hosted Messaging and Collaboration provisioning layer built on top of the Export-Mailbox and Import-Mailbox cmdlets. The work must be performed by an Exchange Administer through the Exchange Management Shell. Hosters and resellers can work with the system integrator or service provider to build a self service that is much more hosted organization friendly.