On February 16th Windows Server 2008 R2 SP1 will be available for MSDN and TechNet Subscribers as well as Volume License customers. On February 22nd, Windows Server 2008 R2 SP1 will become generally available for folks to download via the Microsoft Download Center and available on Windows Update.
Microsoft Exchange team has already tested the Windows Server 2008 R2 SP1 on below version of exchange. All the below version of exchange are supported to run on Windows Server 2008 R2 SP1(RTM version).
Also note that the exchange 2007 was not supported run on Windows 2008 R2 until Microsoft released the Exchange 2007 SP3, read more.
Important - Windows 2008 R2 SP1 includes the hotfixes required to install Exchange 2010 SP1 (listed in Exchange 2010 SP1 FAQ and Known Issues — 979744, 983440, 979099, 982867 and 977020). If you're installing Exchange 2010 SP1 on a server running Windows 2008 R2 SP1, you don't need to install these hotfixes separately.
Ref - msexchangeteam article.