Some major highlights in SP1 as below:
- Archive mailbox can be hosted on different database than primary mailbox.
- In cross-premise scenario, archive mailbox can be hosted in cloud.
- We can directly import/export PST data to/from archive mailbox.
- There are MRM related changes, documented at http://technet.microsoft.com/en-us/library/dd297955(EXCHG.141).aspx
There was another feature supposed to be included in SP1, where Outlook 2007 will be allowed to access archive mailbox. However this functionality will be available in first half of 2011.