Hehe that would work, and is in store. I had something a little more subtle in mind. I remember Seiko selling "Kinetic" watches with internal counter weights. Maybe it's possible to scale up a similar mechanism in a phone case (granted you accept the doubled width) and enjoy the free energy. Of course this came to mind when running yesterday.
Youtube has some videos about a thing called ampy, a tiny case you attach on you. I'm not sure it made it to market though.