Guys/Gals. (I'm talking about United States pricing models here)
You forget the main difference (in terms of buying no contract iPhone 3g (circa July 2008) vs. no contract (iPhone 3g S circa June 2009)
July 2008.... ATT/Apple did not allow ANYONE to purchase the iphone 3g for the no contract price. That's why you saw those crazy Ebay/craigslist prices last July 2008 because those people who wanted to pay full price for the iphone 3g couldn't. ATT would only service customers who wanted to start or extend their contracts. ATT only started offering no contract iPhone 3g pricing since I believe March/april 2009.
June 2009...ATT/Apple allows for no contract price of the new iPhone 3G S for $599/699.
So unless people are completely stupid (yeah I know there are quite of few stupid people left in the world), I wouldn't pay more than $599/699 for the new iphone 3g S (plus taxes) which would mean I wouldn't pay more than roughly $630/740 for the new iPhone 3G S when taxes are included.
And, ATT does not charge an $18 "upgrade" fee when you pay Full price/no commitment price for the phones. If you accidently ever get charged after paying full price, they will waive it immediately if you question it..