US has always been way behind with "cellular" phones (they couldn't even think of a good name for them!)
You can get any phone unlocked in Thailand, I doubt that the US phone company would be interested in pursuing the matter if you simply return to using your contract on returning home?? How long IS your contract? How much discount do you receive on the retail price?

Unlocking in UK is no problem - or you could simply buy a cheap phone outright when you get there