1) buy an US at&t 3G PAYG simcard off ebay
2) once your flight has landed, try topping up your phone, you'll have to go through to an operator and explain that you don't have an american credit card and you would like to use your own.
3) get phone topped up with X dollars (i would put something like $50 on it).
4) go on the internet and sign up for 100mb worth of usage - this knocks off $20 off your running balance on your phone.
5) i had problems with my iphone 3g initially but after following these instructions:
http://www.iphonedownloadblog.com/2...re-not-subscribed-to-a-cellular-data-service/ - it worked fine.
6) congrats, you now have data on your phone.
This is how i did it, i'm pretty sure at&t are the only sim card provider in america, as verizon only provided simcardless phones.