My friend works for Att and he said you only come up on their radar if you go over 5gb a month in data. they will then asess the usage and apply the approprate bil for the overage. now 5gb of data is the industry standard for the so called "unlimited" data plans. Most carriers limit usage to 5gb and it is stated in your data policy. Besides the volume of data my friend said they have gurus who can check the data in the packets sent back and forth to the phone and determine if the data sent cam from a the phone or a computer. But stay under 5gb a month and you should be fine. Ive been doing tethering for about 5 months and no probs.