Suppose there's a qualitative duplicate of the actual world (It might be a world with haecceitistic differences from the actual one, but it doesn't have to be). Call the actual world A, and its duplicate, B.
I'm conscious in world A. Call the extension at the actual world of the things which are conscious S. There are cauliflowers in world B. Call the extension at B of the things which are cauliflowers, S*. Now consider the gruesome intension cauli-consc, which has S as its extension at world A, and S* as its extension in world B (it doesn't matter what its extension is in other worlds: maybe it applies to all and only conscious cauliflowers).
Is there a property that things have iff they are cauli-consc? So long as "property" is intended in an ultra-lightweight sense (a sense in which any old possible-worlds intension corresponds to a property) then there shouldn't be an trouble with this.
However. Cauli-consc is a property that doesn't supervene on the pattern of instantiation of fundamental physical properties. After all, A and B are alike in all physical respects. But they differ as to where cauli-consc is instantiated.
Cauli-consc is a property, instantiated in the actual world, that doesn't supervene on physical properties! Does that mean that the fact that I'm cauli-consc is a "further fact about our world, over and above the physical facts" (Chalmers 1996 p.123)? That is, do we have to say that, if there are such qualitive duplicates of the actual world, then materialism is shown to be wrong by cauli-consc?
Surely not. But the interesting question is: if some properties (like cauli-consc) can fail to supervene on the physical features of the world, what is that blocks the inference from failure of supervenience on physical features of the world, to the refutation of materialism? For what principled reason is this property "bad", such that we can safely ignore its failure to supervene?
Here's a way to put the general worry I'm having. Supervenience physicalism is often formulated as follows (from Lewis, I believe): any physical duplicate of the actual world is a duplicate simpliciter. But if duplication is understood (again following Lewis) as the sharing of natural properties by corresponding parts, then to get a counterexample to physicalism you'd need not only to demonstrate that a certain property fails to supervene on the physical features of the world, but also that some natural property fails to supervene: otherwise you won't get a failure of duplication among physical duplicates. The case of cauli-consc is supposed to dramatize the gap here. Sometimes it looks like you can get properties which fail to supervene, but which don't seem to threaten materialism.
However, when you look at the failure-to-supervene arguments for dualism, you find that people stop once they take themselves to establish that a given property fails to supervene, and not, in addition, that some natural property does so (For example, Chalmers 1996 p132 assumes that it's enough to show that the 1-intension of "consciousness" fails to supervene, without also arguing that it's a natural property) .
Now, I think in particular cases I can see how to run the arguments to address this issue. Add as a premise that e.g. the 1-intensions of the words of our language supervene on the total qualitative character of the world, so that we're guaranteed that if there's a world in which "1-consciousness" is instantiated and another where it isn't, those can't be qualitative duplicates. If now we find a failure of 1-consciousness to supervene on physical features of the world, we'll be able to argue for the existence of physical duplicate worlds differing over 1-consciousness, we now know can't be qualitative duplicates. (In effect, the suggestion is that the sense in which cauli-consc is bad is exactly that it fails to supervene on the total qualitative state of the world).
That all seems reasonable to me, but it does start to add potentially deniable premises to the argument against materialism. (For example, I'm not sure it should be uncontroversial that consciousness supervenes on the total qualitative state of the world. Is it really so clear, for example, that there are no haecceitistic elements to consciousness: that a world containing me might contain a conscious being, but a qualitiative duplicate containing some other individual doesn't?)
So I'm not sure whether the elaboration of the Zombie argument for dualism I've just sketched is the way Chalmers et al want to go. I'd be interested to know how they have/would respond (references welcome, as ever).