Tried two different trade-in options for my iPhone 16GB 3GS in excellent condition with cables and original packaging :
Gazelle
http://www.gazelle.com/
Quoted me $226, emailed me a FedEx Ground Airbill and offered to send me a shipping box free of charge as well. Once they receive package and inspect it payment takes about a week. They will mail you a check or deposit into your PayPal account.
Best Buy Trade-in Center
http://www.bestbuytradein.com/bb/
Quoted me $269 Best Buy Gift Card or $175 check.
Best Buy offers two options:
Instore Trade-in: You print out a Customer Item Tag and bring that to the Geek Squad Precinct counter to pickup Best Buy Gift Card immediately or they can email you check with 7 days.
Online Trade-in: They email you a prepaid airbill, you ship to them and they mail you check or Gift Card in 14 days.
Since I am having my new phone shipped to my house on the 24th, I chose the Gazelle mail in method. Since I qualify for an upgrade @ AT&T I will get the 32GB iPhone for $199 via credit card and the $269 from Gazelle will be automatically deposited to my account via PayPal within a week of shipping it to them.