An interesting short description of common touch-screen technologies on by AP described by Peter Svensson:

" Resistive (Palm Treos, HTC phones and the Samsung Instinct.): Two layers of clear conductive material lie on top of the display. Pressing them together makes current flow between them. Resistive displays are cheap and can be used with a simple plastic or metal stylus, but are prone to damage because the sensor is on top of the display.

Projected capacitive (Apple iPhone and the LG Prada): this touch sensor can lie underneath a protective sheet of glass, making it more durable. The mere proximity of a finger or other object of similar size changes the electrical properties of the sensor's conducting layers, which is why the iPhone is so good at sensing light touches and quick swipes. Projected capacitive sensors can register more than one touch at a time.

Surface capacitive (ATM, kiosks): Like resistive screens, they usually need recalibration, and because they're mounted on top of the display glass, they're prone to damage and wear.

Surface acoustic wave (ATM, large screens): these touch screens vibrate very rapidly. Sensors pick up how the touch of a finger affects those vibrations. The screens can be crisp and clear, but the sensor can't be sealed against the elements."

Why do I blog this? a quick and dirty overview only to be aware of the field.