I have read several places that the iPhones come unlocked, even if you buy them from/for a specific provider. Is this true? What about if you buy for a specific network provider but from a 3rd party like Best Buy or Walmart? They all seem to be "world phones" with all the different radio bands. I plan to buy for full price up front to go no contract so I can switch later if I want... and to be able to get a local SIM card for when I travel outside of the US.