Most unlimited data plans I've seen have some sort of throttling clause, basically stating that if you use more than X GB of bandwidth in a month they reserve the right throttle you to Y Kb/s for the remainder of that month.
I think actually that's just T-Mobile. They definitely seem to be the most progressive of the US carriers.
Most carriers either cut off your data entirely for the rest of the month, threaten to terminate your contract, or charge you an outrageous overage fee.
