Need help using a 3GS without data plan

[duncand]

This is an FYI as well as a request for ideas:

As of yesterday, ATT just discovered we were using an iPhone 3GS (not jailbroken) and added the data plan. We got a txt from them saying so.

I contacted them all indignant, and had them remove it, but the would not do it until after I put the sim card back in the old LG phone, which I did. They then removed the data plan and told me to have a nice day.

Here's the back story:

I bought a used 3GS and upgraded it to iOS4. Then I went into the settings and turned off 3G and Edge data.

We thought we were in the clear and used the phone for about a month.

Then they added the data plan.  

So, now I'm looking for a way to get around this. We like the phone but don't need data, as we have WiFi everywhere.

Any ideas on what to try now?

[duncand]

Will changing the APN help out?

(if it's doable on the 3GS with iOS4) ?

[admin]

You cannot stop AT&T from knowing you have an iPhone on a non iPhone plan/data. Any Carrier can know what phones are on their network