I have been Beta testing Exchange 2007 SP2 for many months now along with Exchange 2010 and Microsoft has released the final product as of today. If you plan on deploying Exchange 2010 into your Exchange 2007 envrionment SP2 will be a requirement.

http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=4c4bd2a3-5e50-42b0-8bbb-2cc9afe3216a

Whats new:

http://msexchangeteam.com/archive/2009/05/11/451281.aspx

http://msexchangeteam.com/archive/2009/05/13/451311.aspx