I don't know why people post when they have no idea what they're talking about. It's completely possible to remove the data plan from an iPhone account, even with an iPhone on it.
OP, if you're just going to sell your iPhone and use a regular phone, buy the regular phone, call up AT&T and ask for an IMEI change. They'll put the new IMEI on your account and from there you're not locked in to being forced to get the iPhone data plan. You can then sell your iPhone. The only catch to this is that you'll need to buy a phone outright in order to do this, as you extended your contract to get the subsidy on the iPhone. You could probably sell the iPhone 3G to make up the difference in this though.
To anyone that has an iPhone and wants to use it on AT&T without data. You need to do the same thing. Get a standard phone, call up for an IMEI change and then have the data plan removed from your account. Then take the SIM out of that phone and put it in your iPhone. If you do this, make sure to either disable data in BossPrefs or have AT&T put a data block on your account. You don't want to be accidentally using data.
This same trick also works if you want to do the $15 MEdia Net plan. When you switch to a regular plan, just ask for the MEdia Net unlimited option. The only thing that won't work on your iPhone is Visual Voicemail and since the IMEI associated with your account is that of a standard phone, you won't get flagged. Bonus points if you use a UMTS enabled phone like the Samsung A727 if you live in a 3G area.