Re: Cell phone usage for USA and "maybe" Caribbean
2go, here's the deal. You bought an Apple product, you're not supposed to do anything besides "use" it. Apple and AT&T make sure of the by using a "micro" SIM card in most of their iPhones, so a standard SIM card used in much of the rest of the world just can't be put in there. IF your phone still uses a standard size SIM card, yes, you can put in SIM cards from other carriers and countries and use the phone there. (Not all countries use GSM phone systems, like AT&T, T-Mobile, or most of the EU does.)
An AT&T phone is "subsidy locked" by AT&T but if you have been an AT&T customer and had your phone for more than a year, you can call 611 and they will give you an "unlock code" that allows the phone to be used with other carriers. In order to use that code, you usually need to first put in a SIM card from another carrier, so again, you'll need to check to see if you have a standard size SIM card, or else take the code with you and try it when you do purchase another SIM card.
This time around, size counts. :-)