In case you missed the news, Windows Server 2008 R2 became generally available on October 22. Good news for early adopters: ORF 4.3 + Exchange 2007 2010 works wonderfully on 2008 R2, so if you plan upgrading, ORF 4.3 will be there for you. To recap, the Exchange 2010 RC is supported by a patch and ORF 5 will support the entire line of Windows Servers (up to 2008 R2) in IIS SMTP-only mode as well.
AFAIK Exchange 2007 is not supported on W2k8 R2.
Ouch… now that hurts. Thanks for pointing out, I mixed up the platforms. Exchange 2010 it is.