1. Are you a new ATT customer? If you cancel within 30 days, you HAVE to return the phone or get charged the full price of the iPhone (not the 399/499) but the 599/699 pricing.
2. If you are just upgrading your line and it's past 30 days and plan to cancel the contract, than you pay ATT $175 ETF.
3. If you are simply planning on keeping your line and using your old GSM phone, than you pay ATT nothing. Just keep on paying for the voice portion of your monthly ATt bill. Just pay for the first month's iphone data plan. Call them up, tell them you aren't using the iPhone 3g anymore, give them the IMEI number of the current GSM phone you are using and they will cancel your $30 iphone data plan also.
iPhone 3g requires $30 iphone data plan. So if you are not using the iPhone 3g, you are not required to have the iPhone data plan on your bill and you can cancel it if you no longer are using it.