Skinput is based on an armband straddling the wearer’s biceps and detecting the small vibrations generated when the user taps the skin of his arm. It measures the vibrations and differentiates them based on bone densities, tissue mass and muscle size.
You can use different parts of your arm and even fingers to interact with it.
According to them it is 95% accurate and you can combine it with a pico projector and get some display on your arm as well.
check out the video
definitely an interesting concept, but until it is integrated into my T-shirt I doubt I will be interested in wearing an armband.