Starting with Xamarin.iOS (previously called MonoTouch) 6.0 several platform assemblies started to be signed (monotouch.dll, MonoTouch.Dialog-1.dll and OpenTK-1.0.dll). This is a problem if you upgrade to Xamarin.iOS 6.0 and you're using third-party assemblies compiled with previous versions of MonoTouch. In this case the third-party assemblies would reference a non- signed version of the signed assemblies, and your project will not compile anymore.
This program will rewrite these third-party assemblies to reference the signed platform versions instead.
Just open the solution in Xamarin Studio and compile.
This is a command line program, so after compiling the project, you need to open a terminal window, navigate to the project directory and execute:
mono iOSRefUpdater.exe /path/to/third/party/assembly.dll
and the third-party assembly will be converted in-place.