I have an iPhone that I use in the US with AT&T's 'GoPhone' plan (no contract). When I go back to the UK, I want to switch, get an O2 SIM and use it there. I emaild O2, who said I would not be able to do this, as the US iPhone will not operate with an O2 SIM. Not sure if this is widely known, but it might help someone. Of course I can hack it, which may be what I do. Either that, or sell it on Ebay and buy another one in the UK.