This isn't a problem with code... App.xaml.cs never gets loaded - so there's no way I can debug it.
If this happens to you, then here are my suggestions:
1. Check that the App.xaml file is marked as ApplicationDefinition and not as Page
2. Check that the App.xaml.cs class is set as the startup object in the project properties
3. Check that the App.xaml does not contain any corrupt entries in Application.Resources (this feels wrong... but I've seen issues with it)
4. Check that the project properties - for namespace, assembly, etc - are all their "normal" values
Hope this saves someone out there some swearing :)