You only have to return it before the the first 30 days are up if you cancel. After the initial 30 day "trial period" you pay the ETF if you decide to cancel your service with AT&T.
As alphatectz said above, if you cancel within 30 days, you have to return the iPhone 3GS, but if you cancel anytime after that 30 days you will have to pay the Early Termination Fee, but you get to keep the iPhone 3GS.
As alphatectz said above, if you cancel within 30 days, you have to return the iPhone 3GS, but if you cancel anytime after that 30 days you will have to pay the Early Termination Fee, but you get to keep the iPhone 3GS.