Microsoft supports installing Exchange 2010 on Windows 2008 R2 SP1. Read my previous article to know more. I had blogged in the past about the need to install many hotfixes on top of Windows 2008 R2 before you can install Exchange 2010 SP1.
Things are better with the release of SP1 for 2008 R2. All the required hotfixes (which we had to install manually) are included in 2008 R2 SP1 & this has made the installation of Exchange 2010 SP1 much easier!
Well done Windows Team!