Don Norman just published a very interesting article on natural user interfaces, here's a quote Gestural systems are no different from any other form of interaction. They need to follow the basic rules of interaction design, which means well-defined modes of expression, a clear conceptual model of the way they interact with the system, their consequences, and means of navigating unintended consequences. As a result, means of providing feedback, explicit hints as to possible actions, and guides for how they are to be conducted are required. Because gestures are unconstrained, they are apt to be performed in an ambiguous or uninterruptable...