My girlfriend brought her iPhone to the US last summer. We asked at a few places about pre-paid cards, but no one had them.
What I did was buy a dirt cheap, pay-as-you-go phone . The phones themselves are as cheap as $10 (TracFone, GoPhone). It's really the total opposite of an iPhone. But, if you won't use the phone that much, it might be good since you won't be home long-term.