In my last vacations roaming costs always became quite high. This year I got a recommendation to buy a AT&T Prepaid SIM card. You can even do this from your home country. In my case from Germany I can highly recommend: BOHL-SHOP.COM. They have a perfect webpage explaining in detail (German) how to order, activate and charge the prepaid micro SIM card from Germany. Before you start your vacation you thus have a ready to use SIM for using your mobile on your USA vacation.
I used the Email-Hotline and got additional detailed support and infos to setup APN settings for using DATA traffic (Internet +email) on the Iphone. Fantastic to find such shops with such a great support online. The SIM card by the way aarived within 2 days after online order.