AT&T has refurbished 3G iPhones up for sale on their site. The 8GB black sells for only $149 with a new 2 year contract, and both the white and black 16GB versions are $249 with contract.
A "refurb" is defined by AT&T as:
previously owned devices that have been unused or lightly used and returned during the 30-day trial period. Each refurbished phone is independently quality tested and loaded with the latest software to meet current factory standards. Some refurbished iPhone 3G devices will have minor scratches.
AT&T (thanks, James)