There are two main types of touchscreens used on mobile devices today -- capacitive and resistive. The resistive touchscreen allows for stylus input while the capacitive supports finger input and doesn't work with normal stylus input. Cypress has announced a new screen tech that adds support for a stylus to a capacitive touchscreen.
The new Cypress tech is part of its TrueTouch line and adds accurate stylus input support with accuracy to 1mm. Cypress has a demo of the screen in action accepting input from a standard pencil. The new tech will let capacitive screens accept handwriting and support for things like written Asian characters.
Some capacitive screens support stylus input already, but accuracy is limited to 3 or 4mm for large icon selection only. The Cypress screen is not only much more accurate, but it is scratch resistant and very durable. The stylus tech is expected to be available widely in Q2 2010.