Last time I was in the UK, I bought a SIM from 3 and one of the features I didn't realise at the time was that I could use my phone abroad with that SIM exactly as I had in the UK. After I finished in the UK, I flew to Denmark and when I fired up the phone 3DK welcomed me to their network allowing me to use my unlimited UK data and also call and be called from UK numbers as if I was still in the UK. This was amazing based on previous experience with roaming from country to country. I drove over to Sweden and the same thing happened. Now that's all well and good, but what about the US? Turns out, when I landed in LAX, I was welcomed to AT&T's network and my unlimited data continued to work (3G only but still, not bad) and then I flew on to NZ and found myself on 2Degrees and still the phone and data worked. If you're not already with 3, I would strongly suggest switching. You can always buy a local cheap phone in the US for doing calls to US numbers, or do as I did and get some Skype credit which will go over the data.