The beta for Exchange 2007 SP1 has been posted to MSDN and TechNet for download.  Weighing in at a hefty 717MB, it looks like the Exchange Team has been busy.

You will of course want to limit this beta to a dev/test environment, while we work the bugs out prior to release.

I have covered the new features of the Service Pack here, and you can download the official release notes here: http://download.microsoft.com/download/5/e/6/5e672458-592a-44a2-b489-11cec19d3c82/RelNotes.htm