People make a big deal about tablets. Personally, I don't see them as the next big thing. Do you think Apple is straying away from them, given their somewhat underwhelming sales?
I think MPAirs are the slowest selling laptop. They compete with other Apple products more directly than Ipads. On the the hand, Ipads, are outselling all macs (in units) already and it's still very early days. Even if sales stay put (seems unlikely), they have an incredible new product line. They are a big thing in that sense, at least.
These might be a replacement for macbooks. At the moment 13' macbook is just a cheap alternative to the superior 13' macbook pro, it isn't best for anything but your wallet. That seems to make it unique among Apple products. A 13' MBAir is more lighter, quicker and looks different. You can imagine people preferring it regardless of price.
The hardware configuration inside of an iPad is destined to become commodity. Eventually, those things will be like displays and mice. The value for Apple will be in the software and software-ecosystem running on the iDevices of the future. (Also in the cloud.)
More comfortable and natural input and display naturally sell. Such tablets are both!