I suspect the transition will be fairly slow. I'd give it another year before DDR3 prices begin to spike... especially considering they're pretty high to begin with.
One interesting catch: I don't think anyone's offering a transitional alternative. For the last step, you could run a Phenom II in an older board with DDR2, or configure the Intel P4x/G4x chipsets with either DDR2 or DDR3. This meant the market could move in "half steps" depending which was cheapest... with this, it's everything or nothing.