So the iPhone [...] was actually an afterthought that came into being while Apple was drawing up plans for the iPad.
"My God, I said, this would make a great phone," Jobs recalled. "So we shelved the tablet and built the iPhone."
This makes a lot of sense. I never could see how anybody could make the mental jump from a normal cell phone to whatever-the-heck the iPhone is. But it makes sense when you know they started with the tablet, because all that technology is clearly meant to be on a much bigger screen than an iPhone.