I agree that renting a cell isn't worth it for an American. Foreign visitors sometimes get package price breaks on car rental + phone.
Be careful with roaming charges on some prepay services. Tracfone charges 4X the usual rate for roaming. Since Cingular bought AT&T, the AT&T phones are now Cingular, and Cingular's prepaid option is not a good one; they charge a fortune for roaming: $1.95 per minute, and you can get no incoming calls outside your home area. (You can setup the phone w/ Orlando as your home area, but then you'll end up paying roaming once you go home. You can change your home area, but there is a limit on how often.)
I've used several prepaid services, and have now settled w/ Virgin Mobile. They work (incl. incoming) in any large city and around all interstate highways, and there is no roaming charge. The price on the domestic service is flat no matter where you are, $.25/minute for the first 10 minutes you use in a given day, then $.10/minute thereafter for that day. Even DH has switched over, it's cheaper than the regular monthly service he used to have. You can set up an account w/ a CC so that your minutes are automatically topped up by a set amount every time you get low. You can keep one of these accounts alive for $80/yr. + the cost of the phone. (You can buy older Virgin phones for just about nothing on Ebay; these services are used mostly by kids, who want to have the coolest new phones when they come out, so they sell the old ones. All you have to do to re-activate the phone is call or go to the website and get a new number.)