It’s now available to all MSDN and Technet subscribers. Some of the great enhancements are;

– Standby Continuous Replication (SCR)

– Exchange Managment Console GUI configuration for POP/IMAP, Public Folders, Send As/Receive As, Delegation

– Move mailbox Wizard

If you would like to know more information about what is included in SP1, check the MSExchange Team’s blog post.