What is important to note here, is the actual unit growth by Apple from Q3/10 to Q3/11. Why is that?
Well, obviously the iPhone, a new iPhone, is one, if not the most highly anticipated tech gadget. So it sees huge sales immediately when it is released.
When we are comparing iPhone unit sales between Q3/10 and Q3/11, we are comparing the first quarter of the iPhone 4 and the fifth quarter of the iPhone 4. So Apple managed to sell more units of a more than one year old phone, than it sold the quarter it was released. And they didn't do it by reducing the price, they still sold at the same full price.
Nobody but Apple can do this. And with keeping the older iPhones around for years, slowly but surely they are also getting into the market that demands lower priced phones. They are simply doing it the Apple way, they don't design a cheap phone, but rely on components getting cheaper over time and so bringing down the prices for these devices.