Here's a starting point for some information:
www.telestial.com www.planetomni.com
If you get a cell phone from a US vendor, you'll need to make sure it's a tri-band (or even quad band) so it can operate on the proper frequency. You'll also need to set it to make sure it works on that frequency . . . I have an older Motorola and it has an option to automatically detect the proper operating range (nothing more frustrating than wondering why you can't get your cell to work only to discover it's set for the wrong frequency, DUH).
If you get an unlocked phone, it's a simple matter to just drop in a new SIM card from a company like telestial. I've used their SIM cards and have never experienced a problem.
Depending on where you're going and for how long, you might want to consider renting a phone once you arrive. Still another option, depending on which cell phone provider you use, is to enable worldwide roaming; that way you should be able to use your phone (assuming it can cover the right band) when you are overseas. The downside to this option is it is often pretty pricey.
I'm sure others who have more experience can provide more info. Check out the links above; they should provide you with some useful information.