Exchange 2007 SP3 – Support for 2008 R2 has been added. MS Exchange Blog.
From the MS Exchange Team Blog:
Exchange Server 2007 is not supported on Windows Server 2008 R2. However, Exchange 2007 SP1 RU9 and SP2 are supported against Windows Server 2008 R2 Active Directory servers. In addition, Exchange 2007 SP1 RU9 and SP2 are supported in Active Directory environments whose domain/forest functional levels have been raised to Windows Server 2008 R2.
From MS Technet:
