Followup: Upgraded my iPhone from 3GS to 4 for MINUS $40.00
Following up from yesterday's post, in fact, I was able to get $300 for my mint condition iPhone 3GS on Craig's list, and was able to buy a new iPhone 4 from Radioshack for $260 after tax!
So, if you have an iPhone 3GS in good condition, and your AT&T contract is up, then you should upgrade to an iPhone 4 and make $40 in the process. Cool!