If your Windows Phone 7 project isn’t starting at all – you’re not even hitting App.xaml.cs, you’ve probably renamed the app project.
Fix: Set the startup object on the app project’s properties page
In the .csproj file, this corresponds to the SilverlightAppEntry element:
I would call it a bug, that you cannot rename a project and have the SilverlightAppEntry update accordingly. But luckily, the work-around is simple.
Have a nice day!