Two player tennis game on mobile phones using AR.
Two phones across a shared set of fiducial patterns and Bluetooth communications so they both know where the ball is.
Studies were done to see if the AR aspect (seeing the opponent) was an important factor. Results supported my initial gut feeling of "Well, yes of course"; although having played it I now have suspicions that a better designed interface could reduce the importance of seeing the opponent in the video stream.
Why? Due to the small screen and the shallow angle all I could see was the opponent's phone, something that could be visually represented if you did not have a video feed in the background. (Apparently this feature was implemented at some other time but not part of these studies.)
In my experience if you believe what you see to be controlled by a human then that's provides you with 90% of the experience. True it might be even more fun to see the person too, but in respects to AR Tennis you can do that in high quality, wide-screen stereoscopic vision by simply glancing directly at the opponent sitting opposite.