Horace Dediu at Asymco notes that the iPhone is now available as five different variants with 10 different price points.
Including all the pricing options It's a very regular pattern. It reminds me of the iPod which, at one time was sold at every $50 increment point from $50 to $500. The logic behind this pattern is that "there is now an iPhone for every budget". Of course, as with any purchase decision, each of these price points has trade-offs. Considering product performance or in terms of option value, the buyer has many things to weigh.
Check out the chart below. Notice how perfectly staggered the price points are...