I'm potentially purchasing a U.K iPhone and I want to make it work in the U.S. I know the radio in the phone will be able to work as it's quad-band but I'm wondering what I'll have to do to the iPhone in order to make it work on the at&t network. Is there anything significantly different between the iPhone U.K. and the iPhone U.S. operating systems? Will it just work with my at&t SIM card without any hacking? Thanks in advance.