I got some very useful comments on my post about how to make XNA games use the full screen of the Windows Phone. The thing that I didn’t make clear was that in an XNA game you will lose screen space even if the bar is not displaying anything. The best way to view the effect of this is to go into the settings of the phone (or emulator) and change the Background to light (Phone>Settings>theme). Above shows what you get. The status bar is drawn white and the game screen area is scaled down by that lovely hardware in the phone itself. As you can see, your game is losing out on screen space.
If you want to make full use of all the screen you just need to add this line into the constructor for your game:
graphics.IsFullScreen = true;