As I think about this, I am visualizing a very faint background watermark/wallpaper layer showing labeled keys, buttons, etc. which could change state when tapped with fingers from the back of the device. I could imagine the layout of such a layer being changed for different application/usage contexts and be visible but unobtrusive behind the main display elements of an application. This could explode the degree of interactivity and the options for interactivity if thoughtfully designed. Plus, it would be very novel.