Is this known yet? We are currently in the planning phases of a Server 2012 R2 deployment.
A comment was made here: http://social.technet.microsoft.com/Forums/exchange/en-US/641963f8-da29-4753-9bb0-8046059627aa/exchange-2013-on-server-2012-r2
That "2012 R2 will be supported with SP1 of Exchange 2013. while we have found no issues running It on CU3 we are still testing it and it will not be supported on CU3 for production use."
I can't seem to find any publicly available information on this date.
belpad