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 Lamborghini on a highway. No matter how fast it accelerates, it can't exceed the speed limit. That limit, however, might increase over time. (The iPhone is Veyron on the Autobahn — it just keeps going faster and faster.)
In other words, the hockey curve on the iPad's growth was unprecedented, even compared to the iPhone. It went mainstream faster than almost any other technology in recent history. Once that happens, you need people to buy more, or you need more people.
I think Jim is right about the buying cycle of iPads being more like 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?