In the first sample of how to add Windows Phone Mango functionality to an existing Windows Phone application, we modified an existing XNA game to leverage the new Silverlight / XNA integration available in "Mango." This allows us to utilize the Silverlight paging model to very easily create a 'pause' screen, that users will get when they return to the application, (after they've navigated away).
Having to navigate away, of course, is a common occurrence on a phone; users receive phone calls, are have to pay attention to external events, (like the street light turning green, for example).
Getting the pause screen to 'work' of course requires more than just adding it in Silverlight. We need to implement tombstoning and FAS, (the whole point of this sample).
Here are some official resources you might find helpful as you consider adding Fast Application Switching to your projects: