AT&T offers subsidized iPhone 3G S prices for certain early adopters
If you already ordered from an AT&T Store, the price will be adjusted when you pick up the handset. If you ordered online through AT&T, you will receive an e-mail and a credit will be issued to you. If you ordered directly from Apple, the manufacturer will determine if you meet the subsidized pricing requirements, and if you do, they will issue a credit to you.
If you're not sure if you qualify or you need more information, you can check out the source link or give your friendly AT&T or Apple rep a call.
Apple iPhone 3G S Specifications