Apple as acquired a patent for a light-sensitive display design. Filed in April of 2006, this doesn't appear to be the same one you find in the iPhone, the MacBook Pro and the first-gen MacBook Air. It's a totally different design that they apparently haven't used, and may not be using in the near future. The image above shows a device that could use the technology.
Below, you can see how the light sensitivity would work, so that one may use a stylus on a touchscreen device much like they do on a Nintendo DS:
Have a look at some of the additional drawings: