More to the point I think smartphone gaming would benefit a lot just from this type of tweak:
A trackball can fit easily onto cell phones without making the device too bulky and can approximate the d-pad/analog is far better than an touchscreen d-pad. It's a physical directional input.
Maybe throw 2 L/R style buttons on the other shoulder -- and while you won't have a gaming platform that neccessarily is exactly like a home console or anything, but suddenly I think you'd have much more playability in games.
For playing say 15-20 minutes of FIFA or Resident Evil or Sonic or Ridge Racer on the go, a set up like this would make games like that (a trackball + 2 physical buttons + whatever on screen buttons the developer wants to add on top of that) far more playable and deeper in play mechanics.
