We have installed TFS2012 (currently as test environment) as upgrade from TFS2010.
We are running unit tests with mstest. They are published to TFS with mstest, too.
In TFS2010 the calculated code coverage is shown.
In TFS2012 the code coverage column (report "Build Summary") is empty. But code coverage is calculated also from the new build server. (.coverage file is existing and has correct values if I open it in VS.) Also build done before migration still shows code coverage in report.
More details (screenshots, etc.) can be send on request.