using gsm technology is the right choice at this time. gsm has capability to roam between one operator to others between one country to anathor country bcoz this technology is built based on global standarisation around the world.
Almost every contry currently use this system for the cellular:http://www.gsmworld.com/roaming/gsminfo/index.shtml
CDMA i think it is better on voice quality but we are stuck if we travel outside the operator coverage. there is few limitation such as thing like roaming as GSM. also in GSM we can just carry the simcard everywhere and change our handset wherever we like. Ithink 3 bigest GSM operator in US are tmobile, cingular and at&t.
big operator using CDMA are verizon and sprintpcs. http://www.cdg.org/index.asp