How do I use a U.S. cell phone in South America without the expensive roaming charges?

I am currently on the market for a new cell phone. I make frequent trips to South America, so I would like a phone that will work in the USA and in South America. I currently use Verizon, and I am happy with them, but with most South American carriers using GSM, I know that I will need to switch to another carrier. Since it doesn't look like the HTC Touch HD or the Omnia HD are coming to USA, the phone that I am leaning towards now is the IPhone. I know I can use the IPhone in South America, but I would have almost a $3 per minute roaming charge. That is too expensive for me. I am looking for a solution to this problem. From some of the answers I have read here, some are suggesting to get an unlocked phone, or have the phone unlocked. But I am not sure what to do. Should I: 1) Buy an unlocked phone? 2) Buy a phone through AT&T, and have it unlocked? If the 2nd option is best, then how can I get the phone unlocked? Once I have the phone unlocked, from the answers I have read, it looks like I would need a different SIM card. How do I get a SIM card that can be used in a different country? Or is there a company that provides a SIM card that can be used in other countries?