Yesterday, I attended the Windows Phone developer camp in London, courtesy of Microsoft. I took the opportunity while there to add trial functionality to my two existing WP7 apps.
I based my code on the information found here:
I naively bound some of my UI elements to be visible when the Trial Mode flag was set. Because I was running in the debugger, and I tried to purchase the application, my app was tombstoned. When the app is resurrected, Trial Mode defaults to true.
However, my debug version prompts as to whether to run in TrialMode or not, and as a result when the app is resurrected, it has already bound to the TrialMode flag. Therefore you must implement INotifyPropertyChanged to broadcast the information.