A few minutes ago I got the following error:

Error    1    API restriction: The assembly ‘file:///C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE\PublicAssemblies\Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll’ has already loaded from a different location. It cannot be loaded from a new location within the same appdomain.

It was weird.

To solve it I removed the two unit testing framework dlls from my project (see below) restarted Vs  then re addeded them back.

    <Reference Include=”Microsoft.VisualStudio.QualityTools.UnitTestFramework, Version=, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL” />
    <Reference Include=”Microsoft.VisualStudio.TeamSystem.Data.UnitTesting, Version=, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a” />

That seemed to work.