overseas to USA, carriers available

[gunsmoke]

Hi all,
I can only think that this is the best location to ask this. I have been using my unlocked phone with Singtel(singapore) and AIS(thailand) sim cards. If I move back to the States, what carriers are available to work with it. the last time I was living in the states (4years ago) Sim cards for carrier were almost unheard of. and trying to switch a phone from one carrier to another was next to impossible. any comments about that?

thanks

[JStraitiff]

t-mobile and at&t are the common ones and its as easy as pulling the sim out and putting in the new one