I can suddenly no longer open .sql files in VS 2012 by simply double clicking in the Solution Explorer. I get the error message:
Unable to open Transact-SQL file in custom editor
I strongly suspect this is after installing SP1 for SQL Server 2012 Express yesterday, as part of Windows Updates. The error dialogue gives me a link to Get the latest version of SQL Server Data Tools, but that link only yields a Page not found on MSDN.
Installing the SQL Server Data Tools - November 2012 update fixed the SQL editor. All is good in the world again.