Re: IPHONE: Travelling to US - prepaid SIM with data available?
I know you can buy pay-as-you-go SIM cards in the U.S. (though not at virtually every convenience mart like the rest of the world). I'm less sure about 3G capability. AT&T, Verizon Wireless and Sprint operate the only 3G networks I know of, and all are closed (contract subscribers only). AT&T, which operates the network U.S. iPhones run on, probably won't let you on with your iPhone. They do offer "GoPhones" which are cheap phones coupled with pay-as-you-go plans, using calling cards. I'm not sure whether international calling is included, either.
Verizon uses CDMA instead of GSM, so your phone wouldn't be compatible anyway. They do offer an "INpulse" pay-as-you-go plan, with international calling available. Sprint also uses CDMA, and as far as I can tell, offers no pay-as-you-go plan.