I think it has to do with that aircraft carrier again. Or more specifically not being able to shift gears that fast.
Right now AMD is producing about 7m+ chips a quarter. Of that only about 1m are k8s. I don't think production is the major hold up, rather getting the OEMs, distributors, Mobo makers, chipmakers etc. etc. all on board takes a lot of time.
Your right, from a cost perspective I'm sure AMD would love to get to all 90nm k8 production as fast as possible. Unfortunately, AMD's business plan makes it a lot more dependent on its partners than INTC is.