That is also assuming that AT&T will have a buy back policy in place for the 3G. It looks like Australia might have this, according to the leaked screenshot from the past few days that shows someone trading in a new iPhone for a BB Bold.
There is speculation and rumor that the trade in/buy back policy may happen, due to the possible flood of handsets that may end up on ebay after the release of the new iPhone. AT&T could then sell the 3G returns as refurbs, or return to Apple for credit.
If it is a buy-back offer, and they just give us the regular discount, I will simply wait one more year till I can get an upgrade AND keep my iPhone 3G (to sell, give to someone else, whatever). I purchased this phone, not rented, and I want something to show for it.