Just a FYI in case others run into this issue. 

Out of the box, my video was docked to the top of the browser window, with a large black region left under the video (the webpage is just this silverlight control, set to width/height=100%).  It is nicer to have the video centered in the middle of the screen, to fix this, I had to download the source code and change the VerticalAlignment of the StackPanel that contains the MediaElement in Page.xaml from Stretch to Center.