Since Windows Phone 8 is not checked as supported target framework in the MEF portable class libraries (currently only .NET for Windows Store apps and .NET Framework 4.5), it will only work in emulator and not on a physical device. So in order to be able to share code between WinRT and WinPRT you have to rebuilt the MEF source yourself, with added support for WP8.
Since the solution is as simple as that I hope there will be a release very soon, with common PCLs targeting all of the above, thus enabling shared assemblies between Windows Store apps and Windows Phone 8 apps for example.
Since the solution is as simple as that I hope there will be a release very soon, with common PCLs targeting all of the above, thus enabling shared assemblies between Windows Store apps and Windows Phone 8 apps for example.