Admittedly I thought that AMD made more sense initially but I think the key might be that Apple isn't really a "computer company" anymore. Laptop and desktop sales aren't nearly as important as iphone, ipod and ipad sales. With that buying AMD may not be as enticing as ARM as there may be more of a future in the small devices. Not sure on the numbers but I would also imagine the margins are much better on the small devices. Small devices are also generally more tightly integrated than laptops/desktops so owning the stack may be advantageous.