I am putting the finishing touches on an app I am working on for the Windows 8 Store. I am opting for a free app that is add supported, however, I noticed that every time the ad refreshed it would steal focus from the only text box on the screen. The solution was simple I just needed to ensure that the following property (IsEnabled) was set to false:

Simple but true, it represents a week of head scratching, hope it helps someone.