Building websites for touch screens requires you to invest a little extra effort into design. The first thing that you must remember is that touch screens are many orders of magnitude less accurate than mice. A mouse offers single-pixel precision, whereas a finger will cover dozens of pixels on the screen. This means that you must build some extra leeway into your user interface to allow people with large fingers to use it comfortably.
The next thing that you must remember is that touch screen UIs have fewer states than normal UIs. You do not have the hover state on a touch screen device, or the ability to mouse in or out of a part of the screen effectively. Touch screen interfaces rely on touch, tap and drag events. This means that you should design your UI so that information is conveyed easily and clearly without the need to use those extra events.
One recent innovation in the world of touch screens is the introduction of multitouch. Currently, multitouch technology is still in the early adopter stage of development, and it has not been standardised. Some devices do support multitouch and touch gestures with up to five points of contact, but there is always the possibility of support for two hands, or different gestures. At this time, multitouch support is something that website designers should be aware of, but not invest too much time into building their websites around. Focus on a clean, simple and flexible user interface and worry about adding fancy technologies at a later date.