Mine did that to me, I refused to pay for it cos 1.5gb of data used isn't worth $400, they were really rude when I told them I was going to go to the telecommunications ombudsman. Went to the ombudsman, out of my contract, no exit fees, didn't have to pay the debts that I had incurred and my phone which was part of the plan was given to me outright.
Call the authorities, they always will help out the little guy.
Check the contract you signed, if it doesn't give a specific amount that they will charge you per extra KB over your data allowance then you will be fine.
Ok cool, didn't know how effective the ombundsman would be against a major company. I'll give it a shot.
Originally I took out a contract and they screwed the tariff up, got charged £300 for two weeks use. They said as the tariff was set up wrong they thought it was fraud and blacklisted my phone. Moved over my contract to a new phone, new sim card etc, didn't ask me to pay that last bill.
I think they're making it up as they go along so they get some of that money back from the fraud cock-up.