i think you should look at things this way. if you can sell your old s939 processor for a decent price, and thus get an X2 processor for a low marginal cost, then i don't think there is much to worry about. Prices on s939 X2 processors (except for teh 4800+) have already started to rebound after the price cuts in July (ie supply is starting to go down, so prices are back up).
I was able to sell my old 3700+ to a friend for a decent price ($75) and then bought myself a 4400+ 89W when they were $220 (now they are $250). So i got the end-of-life upgrade (ie I will never buy another part for this computer; in a year or two I will just upgrade to a whole new system) for my s939 system for a marginal cost of $145 dollars. It was a bit expensive, but I do a lot of photo/video editing and other sorts of multi-tasking, so going dual-core was a good idea for me.
Basically, if you will REALLY take advantage of the dual-core processoring, and can sell your old parts to lower the marginal upgrade cost, then it isn't a horrible idea to upgrade, since i think prices on the s939 dual cores will just keep going up as the supplies run out (you may already have missed the best prices). but for many many people, a 3800+ single core is more than enough processing power to keep them going for at least a year a two. And if you fall into that category, then upgrading would just be throwing money away.