What exactly would Amazon do with the Bitcoins? They can't pay their employees or maintain their servers with it. Amazon would be the only major store taking Bitcoins. They would become bloated and Bitcoin would be worthless. Rick is right, it is too big and too soon. Economies need to cycle money and Amazon wouldn't have anywhere to put the Bitcoins after they have been paid.
They'd convert most of them to cash, which is what everybody else in the world is doing anyway and use them to avoid processing fees to Visa and Mastercard.