Found a pretty interesting thread on a prototype N900 that Nokia had been working on:

http://www.newscientist.com/article/...-illusion.html

Using the touchscreen to create illusions of textures on the screen via electro-static sensors.

Too bad this will never filter down to existing N900 machines I think.. but it seems to be pretty nice technology given that some people (including myself) hate touch screens because of a lack of tactile feedback and not precise enough for the onscreen keyboard taps.