The dual core processors will pretty much require for heat reasons DDR2 memory, so that will involve a socket change.
And there have been dual core processors on the market for several years now. IBM and Sun being the two most prominent examples. Of course, Intel already has dual core processors fabed and in engineering sampling, but I doubt they’ll ship.
Also, AMD dual core processors have one problem: heat. Two Hammer cores right now throw off too much heat and draw too much power for the current socket unless they were quite slow. Intel has a CPU that they could drop in right now and put four on a core at higher performance per core than the fastest Hammer.