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 Transa
Yes, yet another bug in SQL Server 2012 roll eyes
Right click on the SQL file and choose "Open With...". Choose "Source Code (Text) Editor" and choose "Set as Default".
Now it works again.
Installing the SQL Server Data Tools - November 2012 update fixed the SQL editor. All is good in the world again.
Almost a year later, but I found the answer from joerage to be more helpful.
Default to the T-SQL Pane view within Visual Studio 2012 Database Project