All those people carrying around tablets, what exactly are they doing with it? Nothing, nothing at all, maybe some are playing angry birds.
Tablets needs content specifically created for tablets, and right now even with huge app stores out there, genuinely useful apps are in short supply, and even then, do you really need mobile computing beyond your smartphone. The interactivity and connectivity tablets (could) offer with other devices (eg. tell your car to turn the aircon on and get ready for you, or some other home automation stuff) is a good few years away, we're not ready.
Ask anyone who has a tablet if they would buy the next iteration of their favorite device? Maybe Gartner needs to revise their sales targets based on that statistic.