Researchers at Carnegie Mellon University in the USA are working on a prototype system called TapSense, which can differentiate between screen taps from different parts of the finger.
TapSense works by analysing the sound of objects hitting the glass of the touchscreen display. Using an inexpensive microphone attached to the device (the built-in mics don't work), the system can tell the difference between taps delivered by a finger's pad, nail, knuckle, or tip. It can also differentiate between taps from styluses made out of different types of inert materials, such as wood, acrylic and polystyrene foam.
A device equipped with TapSense could therefore respond differently to input delivered by various different finger parts in a contextual way, similar to a right click on a mouse button.