The support documentation for OCS 2007 R2 on Windows 2008 R2 has been released.

Office Communications Server 2007 R2, OCS 2007 or LCS 2005 does not work correctly after you upgrade to Windows Server 2008 R2

http://support.microsoft.com/kb/982020

Supportability is available for Office Communications Server 2007 R2 member server role on a Windows Server 2008 R2 operating system

http://support.microsoft.com/kb/982021

Enjoy