In Intel's case, we're talking about 2.8GHz - 3.0GHz vs. 3.6GHz - 3.8GHz for the high end single core chips. In order to offset the difference, Intel is pricing their dual core chips within about $80 of their single core counterparts. Short of giving dual and single core chips a price parity, this is by far the best approach to assuring dual core adoption.
Why does Intel want to encourage dual core adoption? To guarantee a large installed user base, of course.
So, with dual core CPUs priced very close to their single core counterparts, the choice is simple right?
On the Intel side of things, you're basically giving up 200MHz to have a dual core processor at virtually the same price. But things get a lot more complicated when you bring AMD into the situation.