Microsoft has released Exchange 2010 Release Candidate today. The release candidate is available as VHD and as installable version. The release candidate can be installed on both Windows 2008 and Windows 2008 R2. With this Exchange 2010 will be the only version of Exchange which is supported to run on Windows 2008 R2 for now. Several sources report that Exchange 2007 will not be supported on Windows 2008 R2, even when service pack 2 will be available.
When you already installed the Exchange 2010 beta version you can simply upgrade it to the RC. Besides this Exchange 2010 can be installed in an existing Exchange 2007 or Exchange 2003 environment. In the case of Exchange 2007 you will need to install Exchange 2007 service pack 2 first, this will be available later this month.
When you are getting curious you can download Exchange 2010 RC from the site below.