A company called Canopy has developed this unlikely device, in the form of a case called "Sensus" (the one above is a transparent prototype). It's designed to enable touch controls on the back of the phone – like the Vita's rear touchpad – as well as the sides.
"The sensors can handle up to ten independent responses," marketing coordinator Ian Spinelli tells me at GDC Online. These inputs add to the screen's six, enabling play for "multiplayer or mutant." The idea isn't so much about getting more fingers on the phone, but rather taking fingers off the screen, to avoid obscuring the game.
Spinelli offered examples of possible interactions, including swiping the edge of the case to strafe and tapping to pull a trigger.
"Right now we're doing a private SDK." In the future, developers will be able to download the SDK to add more touchiness for free, though Canopy has yet to determine if the actual developer hardware (a test unit of the Sensus) will also be free.
Update: Canopy informed us that dev kits and the SDK will be free to developers.