In Windows Phone Mango, when the App is deactivated, it puts the App into the dormant state.
This state provides the Fast Switching support where the App is still in memory.
How to find if the Application is returning from Dormant or Tombstoned State in WP7?
When the App returns from the dormant state, the Activated Event is triggered.
The ActivatedEventArgs argument of the Activated Event provides a property called ApplicationInstancePreserved to determine of the App is returning from Dormant or Tombstoned state .