You can dowload the low-encrypted version of this service pack from this site. The high encryption version of Service Pack 1 is not currently available.
Some of the enhancements featured in Service Pack 1 include:
Exchange Routing Objects, a set of components that build on Exchange Server 5.5's event and scripting architecture to simplify the development of routing and approval applications.
Exchange Key Management Server. This version of the Key Management Server has been updated to use the Microsoft Certificate Server as its Certificate Authority.