Microsoft have posted the release notes for Beta 1 of Exchange Server 2007 SP1 at the Microsoft Download Center however the full build has not been made available as of yet.
The following are some of the new features mentioned in the Release Notes:
Outlook Web Access Features:
- Personal distribution lists
- Ability for users to edit server side rules
- Deleted item recovery
- Public folders
- Outlook Web Access (S/MIME)
Unified Messaging Features:
- Support for additional types of UM dial plans
- Additional logic for resolving internal calling numbers
and much more...