Unlike the Europeans, who seem to get some good deals on international usage, Americans get raped. It is very expensive to use American phones overseas. If you're going to do it, I'd suggest getting yourself a multi-band GSM phone and going the sim card route... much cheaper that way. The key is your phone has to be unlocked to accept the different sim cards. Some, I think T-mobile is an example, will unlock it if you ask. Others, I think AT&T is an example, will refuse to do this for you. There are people out there who sell programs to unlock your phone, at your risk, but that is just another expense and hassle. Another thing to consider is where you'll spend your time. Some countries use different systems, Japan and Korea are two that come to mind, and your phone isn't going to work there no matter what. Honestly, unless you really need to be mobile, I'd just go the calling card route. Cheaper and a whole lot less hassle. Hope this helps a little.
cc