Avoid data plan being added onto AT&T iPhone 3GS
You'll have to change the IMEI to avoid the charge. There is simply no other way. As soon a phone connects to a network, the IMEI is registered and that's what carriers like AT&T use to determine if your device is a smartphone.
You can use ziphone to change the number but there's a lot of hurdles and provisos. As far as I know, that's the only JB program capable of doing it. Instructions are outline here: http://m.iclarified.com/entry/index.php?enid=657
The Motorolla RAZR's IMEI: 358472042445412
Another alternative is to go to T-Mobile which don't have the Draconian charge.
Unfortunately you are not the only victim.
AT&T has a TOC that says if you use a smart phone on they network you will have to have a data plan. You do not have a choice here.
Now, you can run but not hide, be aware that some other network providers do the same.
Do not waist your time with Blocking it, or Removing it, or Killing it, or hiding you phone in a cave.
The will eventually charge you for a data plan if you use it or not, if you want it or not. It is a part of they cellular contract.
Sorry for the bad news :(
Sources: http://forums.att.com/t5/Data-Messaging-Features-Internet/Smart-Phone-without-data-plan-Possible/td-p/3522573
http://www.att.com/shop/en/legalterms.html?toskey=wirelessCustomerAgreement#whatAreTheGenTermsApplyDataMsgPlans
The small print:
6.3 What Are The Voice And Data Plan Requirements?
A voice plan is required on all voice-capable Devices, unless specifically noted otherwise in the terms governing your plan.
An eligible tiered pricing data plan is required for certain Devices, including iPhones and other designated Smartphones. Eligible voice and tiered pricing data plans cover voice and data usage in the U.S. and do not cover International voice and data usage and charges. If it is determined that you are using a voice-capable Device without a voice plan, or that you are using an iPhone or designated Smartphone without an eligible voice and tiered data plan, AT&T reserves the right to switch you to the required plan or plans and bill you the appropriate monthly fees. In the case of the tiered data plan, you will be placed on the data plan which provides you with the greatest monthly data usage allowance. If you determine that you do not require that much data usage in a month, you may request a lower data tier at a lower monthly recurring fee.
In English, if you do not get the data plan your self (a cheep one) they will slap you with the most expensive data plan.
Jailbroken phone or not, there is no way around it as long you use the ATT network. Every call and text will broadcast the IMEI number. If not already in they database, AT&T would only need to do a manual check on it.
UPDATE as of March 8, 2014
AT&T says it is cutting wireless data charges for individual customers who have no annual service contract - @Reuters