When you bought the 3GS, they gave you discounted rate, so that tacted on 6 months to your expiration date. My wife's contract was up in Sept, cause she had a 3G and mine was not up until Feb...cause I got the 3GS.
You are not eligible for discounted price on iPhone 4 cause you expire in Jan or Feb, due to 3GS purchase. So had I stayed with ATT, i'd have to pay full price for that new phone. Steph could upgrade for cheap.
So in other words.......PWNED!