On my last couple of visits to USA I was surprised at the cost and lack of value of the "deals" available at the big branded stores. So I stayed with my UK plan whilst in the USA. (Unlike Aus where I quickly found decent deals for pre-pay, a bit like Mr Speedbird's).
So for the OP's needs, I would use wi-fi on something like an ipad touch, or something cheaper, and stay with my old phone for voice calls.
Lots of free wi-fi around - although drinking Starbucks' coffee is too high a price to pay.