[jonnythan]

The screen is a resistive touch screen. It registers touches when your finger physically presses one sheet of material into another sheet. Trying to use multi-touch on such a screen would not be very accurate.

The iPhone screen is a piece of hard glass. It's a capacitive touchscreen. It registers physical contact with any material that has electric capacitance, such as human skin. Since you're not squeezing two sheets of material together, it's much much more responsive and accurate for using multi-touch.

Of course, you can't use gloves or a stylus with the iPhone.