Georgia Tech Mobile ground have developed a collection of mobile phone games that incorporate physical gesture interactions. Users interact with the game by physically moving the phone in free space. The games do not rely on special motion sensors. Instead, they use the mobile phone's camera as the "motion detector".

By using computer vision techniques like motion blur detection and optical flow it is possible to detect up to 6 degrees of freedom. 

They have developed three entertainment applications to explore this application domain. The first is a simple single player "Pong" game. The second is a "Doodle" application that demonstrates the various degrees of freedom. The third is a multiplayer version of the "Pong" game that leverages the Bluetooth capabilities available on many camera phones.

As it is using the camera on the phone to detect the motion, the camera must be at least responsive and fast enough to run the computation.

via www.picturephoning.com