Microsoft has confirmed that the support for running Exchange 2007 on Windows 2008 R2 is in the pipeline and an update should be released sometime next year. It is good to note that the exchange team is listening to customer feedback!
Exchange is supported on Windows 2003, 2003 R2 and 2008 at the moment. Don’t confuse this news with the fact that the support for running Exchange 2007 SP2 in a pure 2008 R2 domain, which is already in place. Read more about the upcoming support in the Exchange Team blog