Jobs gave an interview yesterday (D8 Conference) and noted that the iPhone had its origins in a rough prototype for the iPad (even though the iPad was released almost 3 years after the iPhone).
Here is the quote:
“I had this idea about having a glass display, a multi-touch display,” Jobs told The Wall Street Journal’s Walt Mossberg during the June 1 onstage session. “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 then got … scrolling working and some other things, and I thought, ‘My god, we can build a phone with this.’”
and the video recording: