Not too soon at all. As a teen and broke college student, I can tell you for certain that I could never afford the iPhone. Not because of a $600 price tag, but because $600 is too damn expensive for something that has so little space, and I can't afford the data plan. People younger then I with a part time job could never afford an iPhone. Parents aren't going to get a $600 phone and upgrade the plan for a tween. That's a HUGE demographic that's untapped, and who would never buy an iPhone anyway. If Apple releases a 2nd iPhone, it may cannibalize some sales of the iPhone, but it would more then make up for lost sales by catering to a completely different market that's more willing to spend money.
R&D costs would be almost nothing. You already have the software, just get rid of the internet parts, multi-touch, replace it with a scroll-wheel input, and add an antenna and radio to the nano, and boom. Insta-iPhone nano.
$250 for a 4gig model, $300 for an 8gig model.
the iPhone capacity gets doubled.
iPod Nano's get doubled too. Believe it, it can work out, it's just a matter of whether Apple will do it or not.
P.S. I think it will be a candybar form factor with a click wheel that displays numbers on it in a circle, like an old rotary phone. That's why Apple filed the patent for the touch pad that can display stuff.