AT&T should institute a hard data limit for customer's that allows customer's to choose to turn on a limiter that stops the customer from going over the data tier that they pay for. AT&T clearly has no desire to institute any data rollover plan if you read the forums regarding this.
The only reason not to include this feature would be greed on the part of AT&T, because AT&T wants to charge you for that next GB of Data. Someone from AT&T will invariably reply to this post and cite cost of programming which we know to be a hoax or cost to implement another hoax.
They did offer Data limits through the Smart Limits feature set , that helped you manage data overage on a per line basis but that feature is being "retired " on 9-5-2014.
Come on AT&T give us something useful. The old you have reached 70 percent of your data message just doesn't cut it.