AT&T do prepaid sim cards but watch out for the terms and conditions. I bought a sim December 2012, used it for a month then went back to the UK. Came back to Florida December 2013 and not only did the sim not work because the minutes had expired, AT&T wouldn't reactivate it so I had to buy a new sim and get a new number. Not really a big deal but I've got better things to do with my time than spend it in an AT&T store filling in forms.

Just remembered something else... You can't buy new minutes from outside of the U.S. unless you have a U.S. credit or debit card. The website won't accept foreign cards.