The metrics surround the resale value of this are such that it's just way to difficult to give you an accurate picture of what the market will look like.
-Apple could introduce the 3g owners wet dream of the ultimate iPhone with every feature ever imaginable since this is an "incremental upgrade" (to them).
-If Apple tells ATT to go away and gets into bed with verizon.
-If ATT succumbs to the whining of the 3g owning population and gives all 3g owners the ability to extend their contracts in for a 199/299 3g.
We'll know in about 12 months exactly what the resale value is