I am not surprised that people can sometimes go over their data limits with modern smartphones such as the iPhone.
I was on the Edinburgh Tram system the other day and it offered free wifi, so I connected and signed in. A little disappointed that there was a 20MB data limit on the free wifi
I was astonished to see the data usage on my phone gobble up the data so fast. Within seconds I had already used 3MB, and all I was doing was watching the sign in page.
I am guessing that the phone was doing something in the background, probably checking e-mail, but what this does show is how much (and how fast) smart phones use data. You can start to imagine the bills if you were using your phone abroad or on a limited data contract.