> Again, my sat nav allows you to scroll like this - but it's very 
> unpredictable. It doesn't "respond" very well to the touches you're trying 
> to make. I don't know why. Also, it's very hard to *slide* warm, sticky 
> fingers over a plastic surface in a smooth motion.

Your satnav is likely a resistive touch screen, it works when you physically 
bend the top layer enough to make contact with the bottom layer 
electrically, and then just like a variable resistor you can measure the 
resistance to work out the position you touched.  The iPhone uses a 
capacitive screen, as soon as you touch the surface of the screen the 
electronics can detect the capacitance of your finger, also using clever 
electronics/software it can work out the position of 2 fingers in contact, 
to allow those wacky zoom and rotate operations.

The disadvantage of capacitive touch screens is that they only work with 
your bare fingers, not while you are wearing gloves, or any sort of stylus. 
The accuracy is also much lower than resistive touch screens, but that 
doesn't really matter when you're using your fingers.

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