Almost like an air guitar, but for furniture design. The dream is to wave your arms and have the form you want materialize. Some folks in Sweden have come up with a motion-capture system which outputs to a 3d printer. You, as designer, wave a dongle, and then later you'll get a plastic prototype of your doodle sketch.
Here is how the inventors, called FRONT, describe it:
The four FRONT members have developed a method to materialise free hand sketches. They make it possible by using a unique method where two advanced techniques are combined.
Pen strokes made in the air are recorded with Motion Capture and become 3D digital files; these are then materialised through Rapid Prototyping into real pieces of furniture.
Best to watch a short video on how it works.
What is missing is the step shown in the video. To optimize this process you need to be able to see what you are drawing, as you see in the video. Ideally, you would do this in some kind of Virtual Reality and then exit to RL to see your finished molded sketch.
I believe the idea of doodling as a direct interface to an instant prototype -- as one does in Google's SketchUp -- is a concept with a lot of legs and will come to be one important mode for designers of the future.