I have an iPhone 4 (software v4.3.3, baseband 02.10.04) which I've jailbroken. I didn't bother worrying about unlocking as I'm on a 24 month contract with O2 here in the UK so wouldn't have wanted to use another carrier anyway. Now I'm headed to the USA for three weeks over the summer and would like to keep using my iPhone to make calls, post photos to Twitter etc... I've heard horror stories about roaming prices and data charges for use abroad, so I thought perhaps I'd be better off purchasing a pre-pay US sim, but I don't think I'll be able to do that without unlocking my phone....?? Does anybody have any advice on this?