This was an interesting item to appear recently at CES 2011: the Fling controller for iPad (although I’m sure it will work fine on other touch devices with the right amount of screen real estate). As you can see it just attaches by the two suction cups and allows you to easily interact with games that use virtual joysticks.
Check out the video below to see it in action. I have to admit, while it kind of goes against the whole idea of a touch screen – by adding a physical controller – it does look like it provides a richer experience when playing games. Will this lead to a set of attachable buttons for pressing fire etc? I doubt it, but never say never :p.
If you want to know more about the Fling, head over to Ten1’s site and have a look.