Exchange 2007 will be supported on Windows Server 2008 R2, Kevin Allison, GM Exchange Customer Experience, posted on the Exchange team blog today. With the general availability of Exchange 2010 just around the corner, Microsoft had earlier decided not to update Exchange 2007 to support its latest server operating system. Exchange 2007 is supported on Windows Server 2003, Windows Server 2003 R2, and Windows Server 2008. This change in course is a result of customer feedback.
An update to Exchange 2007 will be released some time next year to enable full support for Windows Server 2008 R2. More in Supporting Exchange 2007 on Windows Server 2008 R2 on the Exchange team blog.