I would love to know how that is even possible. You cannot connect to the network without a sim except for emergency calls.
And if you ARE somehow connecting to the cell network and pulling data, you don't think you IMEI is getting transmitted? even if you are under the wire - so to speak- you will eventually get caught, especially now that you have made this something for lots people to try. I can see a handfull of people pulling this off, but when a couple hundred do it... it's going to get noticed. If its even possible.
Call AT&T and have them cancel all the data network to your phone. I am using my non iPhone plan and didn't want to get charged when using the internet, I only wanted to use the browser while on wi-fi. I called and had them cancel and it worked out great, just make sure to tell them to keep your text messaging on if you use it.