http://news.bbc.co.uk/2/hi/technology/8587486.stm

US researchers have found a way to work out where the tap touches and use that to control phones and music players. Coupled with a tiny projector the system can use the skin as a surface on which to display menu choices, a number pad or a screen. Early work suggests the system, called Skinput, can be learned with about 20 minutes of training. “The human body is the ultimate input device,” Chris Harrison, Skinput’s creator, told BBC News.

Further link to CMU invention: http://news.cs.cmu.edu/article.php?a=1322
Video: http://techblips.dailyradar.com/video/skinput-appropriating-the-body-as-an-input-surface-chi/