With the I phones you have a couple of options, the safest is to make sure you turn off data roaming and 3G connection, this will still allow telephone calls. Make sure that that you do this before you get on the aircraft in the uk, if you wait until you get to the USA it's already to late and the phone will have connected to the local networks and update your apps in a few seconds and that will result in a roaming data charge ,
Wifi is widely available in the USA so if you want to use it go into aircraft mode , this will turn off wifi automatically so you will need to urn the wifi back on again, at the finish of your connection turn off the airport mode and check that the phone still has 3G and roaming turned off,
The other method is to have your phone unlocked o2 do this for free, get a local sim that includes data and use the phone as the locals do, this gives you a USA telephone number and you pay local rates, t mobile do a good pays sim in the USA. It costs 2 dollars to buy the sim card and you can then pay as little as 2 dollars a day for unltd use for USA calls and text although international rates can be a little expensive the data is at 2 g speeds for 3 dollars a day you get the same but at 3 g speeds, you can get a sim at any
Walmart etc and the top cards are available at any supermarket
Also remember you will need a mini sim card which are not readily available in the us or buy a sim card cutter of e bay, about £3 and cut a normal one down. The alt is to search the net there at plenty of travel sims out ther such as go sim etc