Those unlimited data plans will not have this problem (but you can use to keep track of consumption), but we have plans with a certain amount of MB per month we have control how much you eat so we are not overcharged.
Data Access Control is a free app for iPhone, Which allows control the use of data (3G and WiFi). We can see how much we consumed in the days that passed this month, see a projection of what could end up spending, etc.
