Using MEF to link view model locator and load assembly UIs dynamically

So I decided I wanted to create a WPF navigation application. I was definitely inspired by Billy Hollis’ StaffLynx application. The application has to have the following requirements: 1.ViewModelLocator must be dynamic, and the parts satisfied by MEF, including UI Extensions. 2.UI pages in separate assemblies will load based on pack Uri syntax. 3.UI extension dlls cannot know about the main app, and vice-versa. e.g. No direct references. 4.UI Extensions have to be loaded from the ProgramData folder, because applications cannot write directly to the Program Files folder. 5.Must use my favorite MVVM Framework MVVM Light. Although you could customize this for any MVVM framework. 6.Must work in Expression Blend.