AppleInsider reports:

With its U.S. Patent No. 8,581,870 for "Touch-sensitive button with two levels," Apple may be on the verge of introducing a new form of input; one that could change the face of iOS.
While a majority of the patent refers to a "button," a notation buried near the end of the document points out that a touch screen operatively coupled to an actuator can be considered a "touch-sensitive depressible button".