there is obviously no real way to answer this question. Just a few more days and we will all know. W/ the iphones im always one generation behind. I have successfully purchased two (perfect condition) iPhones for $250 over the last 2 years... iphone 2g last year for $100 and iphone 3g this year for $150.
It's worth it to me to upgrade 1 year late, but some people might think its pointless to upgrade at all. Some people might think owning a smartphone sucks. Some people might not have $300+ to upgrade. Some people might say that the Palm Pre is better. Some people might say wait. Some people say extra ram and memory is MORE than enough reason. Some people say Apple is the devil. See where Im going with this?
Bottom line, we don't really know what its going to look like. Should know soon. Better be saving your pennies and checking your AT&T bill for months to come.