Making order forms better

I've filled out serveral order/registration forms online lately and they got me to thinking about the proceess. In the US, every ZIP code maps back to a city or town. Yet I have to fill out both the ZIP code and City/State portion of the form. Why not one or the other? In many cases the server validates that I have the right ZIP code, so they're checking it anyway. Why not eliminate it altogether? I'll post a sample later tonight to see if we can make this type of form a little cleaner.