- Replace "Format Version 12.00" with "Format Version 11.00" (without quotes.)
- Replace "# Visual Studio 2012" with "# Visual Studio 2010" (without quotes.)
Editing the solution file works but you may still get the incompatible error if you had .NET 4.5 selected as the default .NET version in your VS2012 project and your VS2010 enviroment doesn't support that.
To quickly fix that,
Open the VS2012 .csproj file in a text editor and change the TargetFrameworkVersion down to 4.0 (from 4.5).
VS2010 will then happily load the "edited" solution and projects.
You'll also have to edit an app.config files that have references to .NET 4.5 in a similar way to allow them to run on a .NET 4.0 environment.