Apple chief Tim Cook had an interesting line about the velocity of change in his earnings call last week:

…through the last quarter, I should say, which is just 2 years after we shipped the initial iPad, we’ve sold 67 million. And to put that in some context, it took us 24 years to sell that many Macs and 5 years for that many iPods and over 3 years for that many iPhones. And we were extremely happy with the trajectory on all of those products. And so I think iPad, it’s a profound product.