Hi,
it is our intent at Esri to provide full support for Windows Server 2012 with the upcoming release of ArcGIS 10.1 Service Pack 1. The release of Service Pack 1 is scheduled for the week of October 29th.
This note is just to indicate our current plans, although the definitive answer will need to wait until we make an official announcement. Stay tuned, it will not take longer than a couple of weeks.
Thanks,
Ismael Chivite
ArcGIS for Server Senior Product Manager