From Luke Wroblewski on A List Apart and thanks to Arjun for passing this one along. Wanted to get this on the Bivouac in the Interface Hall of Fame as an outstanding example of reducing percieved complexity and how, through great design, we can help pull users through transactions that can otherwise seem overwhelming.
By stripping away the form elements as each section of the form is completed, Apple helps reinforce a sense that users are making progress through the check out process. Along the way, should users leave a required field of information blank, a yellow prompting box appears right next to the field letting them know it is needed to continue. Stratification of information in clear zones serve as landscape "mile markers", letting users intuitively sense where they are in terms of completing their task.