From Job’s session at the D8 conference:

“I’ll tell you a secret. It began with the tablet. I had this idea about having a glass display, a multitouch display you could type on with your fingers. I asked our people about it. And six months later, they came back with this amazing display. And I gave it to one of our really brilliant UI guys. He got [rubber band] scrolling working and some other things, and I thought, ‘my God, we can build a phone with this!’ So we put the tablet aside, and we went to work on the iPhone.”