While the iPhone continues to grow and grow, the iPad seems to be leveling off. Why the difference? Some believe it's buying cycle, others cannibalization from other Apple products, and still others the limits of the addressable tablet market. Jim Dalrymple, writing for The Loop:
It will take a while before we actually know what the buying cycle is for the iPad, but I'm guessing it's a year or two longer than the iPhone. That is clearly going to affect sales of the tablet, but I still don't think it's anything to worry about.
I've always looked at it like this: The iPad is like a Lambo on a highway. No matter how fast it accelerates, it can't go faster than the speed limit. That limit, of course, might increase over time. (The iPhone is on the Autobahn — it can conceivably keep going faster and faster.)
I think Jim is right about people replacing their iPads like they did their traditional computers. I also think Tim Cook is right about tablets continuing to be an area of growth. Over time, more PCs and more iPads will be replaced with... future iPads.
As to the current numbers, it's interesting that the iPad is still doing well despite pressure from larger screened iPhones and more power efficient Macs. Imagine where it can go when it starts to apply some pressure of its own?