Paul Graham:

The iPhone isn’t so much a phone as a replacement for a phone. That’s an important distinction, because it’s an early instance of what will become a common pattern. Many if not most of the special-purpose objects around us are going to be replaced by apps running on tablets.

He basically argues that tablets, such as iPhones/iPads, are going to be replacing a great many things in our lives and that is rather hard to argue with.