"a being that is exactly like you in every respect—identical behavior, identical speech, identical brain; every atom and quark in exactly the same position, moving according to the same causal laws of motion—except that your zombie is not conscious."

As someone with a medical background, I find it very hard to believe this is possible. Not unless Consciousness is reduced to something so abstract and disconnected from what we consider our "Selves" as to render it almost meaningless. After all, traumatic brain injury can alter every aspect of your personality, capacity to reason, and ability to perceive. And if "consciousness" isn't bound up in any of these things, if it exists as some sort of super disconnected "Thinking thing" like Descartes seemed to think, I really can't see the value of it. It's like the Greek interpretation of the afterlife where your soul exists as a senseless shadow, lacking any concept of self or any memory of your past life. What good is an existence that lacks all the things which make it unique?

Then again, as a somewhat brutal pragmatist, I cease to see the meaning in having an argument when it seems to devolve beyond any connection to observable reality.