half of all iOS sales took place last year. And while iOS sales were led in the fourth quarter of 2011 by the record 37 million units of the Apple iPhone 4S that were sold, it is the large jump in sales of the Apple iPad that really helped iOS devices at the cash register. To date, Apple has sold 55 million of its tablets, a figure that surprises even Apple CEO Tim Cook.
Cook noted at a Goldman Sachs conference that while it took Apple 22 years to sell 55 million Macs, it took them 5 years to sell 22 million iPods and 3 years to sell that many Apple iPhones. As for the Apple iPad, Cook says the tablet is "on a trajectory that's off the charts." Speculation is that the next generation of the tablet to be introduced on March 7th with some changes to the design and specs of the device.
source: Asymco via AppleInsider