Microsoft just released SP2 for Exchange 2007 which is required if you are installing

Exchange 2010 in an existing environment. Be sure to test in your lab environment

before deploying to production servers.

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

Exchange 2010 RC has been released and is feature complete:

http://msexchangeteam.com/archive/2009/08/17/451974.aspx