In this quick post, Shawn Wildermuth documents an issue about debugging in
Windows Phone 7.
Source: Shawn Wildermuth's blog
When you are running your application and you want to test it tombstoning (e.g. deactivating), you should notice that Visual Studio stops debugging when that happens. When you return to your applciation (via the back button or in the case of a chooser, once you've choosen something), you'll get a black screen. This isn't a bug...its a feature!