Most of you are probably not aware that AT&T, as well as some other mobile carriers, place a cap on your internet usage. Anything over 5GB of use is considered excessive and, while your service will not be terminated immediately, you will receive a warning. After which, your service is subject to termination if you continue to exceed the “soft” limit. This “Cap” includes all unlimited data plans such as blackberry and iPhone data. This information was printed on your bill some months ago.
Now, AT&T has extended the cap to include pre-paid phones. Only for pre-paid, if you exceed 9,900 KB in one month, you will be billed a flat rate of $99.99. It will be deducted from the account you use to pay for your GoPhone, etc. This information will be sent via SMS and mail throughout the coming months. In this case, since you pay $0.01 per KB anyway, it simply stops charging you after you reach 9,900KB.
You can find more information on AT&T’s Wireless Data Service Terms and Conditions