Exchange Server 2007 SP3 is available for download.
As highlighted in Updates to the Exchange Supportability Matrix this past November, this third service pack for Exchange 2007 enables Exchange 2007 to be installed on the Windows Server 2008 R2 version of the operating system.
Download details: Exchange Server 2007 SP3
As usual, there are two versions : 32 and 64 bit, however, only the 64 bit is supported in Production environment.
SP3 introduces some schema changes!