E2K7 SP1 Beta
The public Beta version of Service Pack 1 for Exchange Server 2007 is available for download at the TechNet and MSDN sites. When I looked at the MSDN site, only the x64 version was available, but the 32-bit version is expected to be published as well.
The Release Notes for the Exchange Service Pack 1 have been published some time ago. Meanwhile, there is some additional information at the You Had Me At EHLO blog (Exchange Server 2007 and Windows Server 2003 Service Pack 2) regarding the requirements for the SP1. It looks like Windows Server 2003 SP2 is required before installing E2K7 SP1.
The RTM version of the Exchange Service Pack is expected by the end of this year, at the same time that Windows Server Longhorn sees the light of the day.