Isn't the iPhone cpu a 620MHz ARM chip? If so, how likely is it that the Silverthorne will find its way into the iPhone? Wouldn't all code have to be recompiled and optimized for the Silverthorne? If that is the case, it makes no sense to issue the SDK only to revamp it entirely for Silverthorne before the year is out.
I don't think the iPhone will see a Silverthorne in the near future or possibly ever. I do however believe that Apple will produce some sort of tablet or true ultraportable (7 to 10 inch screen) device and Silverthorne would be the perfect chip for such a device or devices.
I honestly believe that Apple has a fully functioning tablet prototype but the hold up is the modifications to the software to recognize multi-touch gestures. Evidence of this is the changes made to iPhoto to accommodate multi-touch for the new track pad in the Macbook Air.