Started using the NonlinearNavigationService to solve some issues in my app where the back button threw my app into a loop and the user could never back out of the app to close it. Which made the app fail certification incidentally. (Because you manually can't prune the navigation history, this is really the best workaround to avoid this type of loop.) This service works great -- just add the dll and you are good to go -- except it doesn't work with the Page Transition Service from the Silverlight Toolkit.  That's too bad, but hopefully someone out there will address that! If anyone does, let me know...