As reported on the Microsoft Exchange Team Blog, Service Pack 1 for Exchange Server 2007 has been released! The Microsoft Exchange Server 2007 SP1 SDK is also available online and for download. If you want to find out about the changes for developers in Exchange 2007 SP1, including additions to transport extensibility, Exchange Web Services, and Outlook Web Access customization, check out the What's New in Exchange Server 2007 SP1 Extensibility article.
There's a lot of good stuff in SP1. Go learn. Go develop!