For those interested in the Apple iPhone 3GS, a refurbished 8GB model is $29, a 16GB model costs $49 while a 32GB unit is $79. All require the signing of a 2 year pact. Finally, an Apple iPhone 3G with 8GB and a cosmetic blemish will run you just $19 with a 2 year deal.
All of the refurbished Apple iPhones, whether the iPhone 4 down to the iPhone 3G, also require you to pay a monthly data charge of at least $15.
For your piece of mind, the carrier has tested the phones and has re-boxed them. An AT&T rep says that Apple will honor what is left of each unit's original 1 year warranty while AT&T has a separate 90 day warranty on each refurbished Apple iPhone. For more information, check the source link.
source: AT&T via 9to5Mac