I believe the astonishing growth rate of the smartphone market is proof that smartphones will very soon will outsell PC's. The mobility a smartphone can bring and the way smartphones have evolved in performance in the past year and will keep evolving from now, is also evidence that consumers won't need much more than a smartphone in just a few years from now.
Perhaps smartphones are not yet the best productivity tools, but this is only because it's limited by the small display, which can change in the future, either by drastically improving the virtual keyboards, or by improving voice commands further.
Tablets are going to increase the smartphone OS's popularity, while in the same time they are going to help decrease our dependence on full desktop OS's like Windows and Mac OS. This is why Android has the big opportunity to become the most used OS in the world in 5-10 years from now.