Well...the value is definitely is not going to go up, thats for sure.
However, the resale value has been pretty consistant, netting you anywhere between $200-$350 based on condition/unlocked etc for either the 3G or 3Gs.
I would think it might go down a little, especially since they will be $100 less than they are currently...but there are still plenty of 3G iPhones out on the market, so who knows.