In WP 7.1, a BackStack property was added to the navigation service that provides an enumerable of navigation history. Without this property, it is not possible to reliably remove all items from the backstack, which is necessarily in a user logout scenario (back button should exit application).

Footprint is:
public IEnumerable<JournalEntry> BackStack { get; }

Full class documentation is here:
http://msdn.microsoft.com/en-us/library/system.windows.navigation.navigationservice(v=vs.92).aspx