When I travel in the US, I turn my data roaming off.
Recently in LA, I would switch data roaming on periodically to look up directions, do some work emails etc, then turn it off again. My data roaming alone for simple things like that was $300.
There was an article in the Winnipeg Free Press, about a guy here who kept his roaming on and checked emails etc and his bill was $18000! My friend of mine also had a $10000 bill once when the iphone first came out and he didn't shut data roaming off. Be careful!!!!