Mobile computers are popular because they’re convenient. One day we looked up and realized we were mostly using our iPhones and not using our laptops all that often anymore.
Mobile computers (smartphones and iPads) are cheaper (you can get an iPhone 4S for $0), they’re always nearby (mine’s usually in my pocket), always connected to the Internet, and they’re easier to use for casual computing like checking in on email and social networks, jotting down a few notes, and checking off a few tasks or adding a few new ones (which are all the computing needs of most people).
P.S. I read David’s article and then wrote and posted this link all from my iPhone.