It's a question that's only come up recently, in the age of touchscreens and 3D: how do you manipulate a virtual 3D object via a 2D touchscreen?
Well, one way is to use Beyond, the new tool out of the Tangible Media Group in the MIT Media Lab. Basically, it's a tool that when pressed against a touchscreen, extends virtually into the 3D space. This lets you act as if it's in that space, drawing, pushing or adjusting as you wish.
It's a clever idea, one that is sure to become more useful as 3D touchscreens become more prevalent.