I would like to be able to press a button to disable the menu bar in my application. If you go
menubar.IsVisible = false;
it throws a null refe
You can access this as a static member on ApplicationBar. i.e.
ApplicationBar.IsVisible = true/false;
Some background here on why ApplicationBar doesn't play like normal silverlight elements.
Why are the ApplicationBar objects not FrameworkElements? - Peter Torr's Blog