Rather than continue with my critique, I want to say something in support of the paper. Much of the criticism levelled at it so far in this thread—including my own—is coming from even further to the Left, so to speak, and thus might seem a bit esoteric to your old-fashioned dyed in the wool Cartesian or empiricist to whom these ideas are entirely alien. The fact is that the paper's thesis remains bold and exciting, because it goes against the philosophy of mind which in the twentieth century became common sense, held unquestionably by many if not most educated people and certainly most people working in cognitive science. That I am my brain, that my head is the locus of my mind, that my body parts are appendages to the all-controlling soul-in-the-head: this is what Clark and Chalmers are up against. And even if they're limited by their own commitment to cognitivism, they manage to question some of the most fundamental prejudices operating within it.
There's a moment in the paper that reminded me of Merleau-Ponty's admission that empiricism—for which perception is just the result of the physiological processing of raw sense inputs—could not be decisively refuted, his phenomenological re-descriptions always being open to being explained away in empiricist terms:
By embracing an active externalism, we allow a more natural explanation of all sorts of actions. One can explain my choice of words in Scrabble, for example, as the outcome of an extended cognitive process involving the rearrangement of tiles on my tray. Of course, one could always try to explain my action in terms of internal processes and a long series of "inputs" and "actions", but this explanation would be needlessly complex. If an isomorphic process were going on in the head, we would feel no urge to characterize it in this cumbersome way. In a very real sense, the re-arrangement of tiles on the tray is not part of action; it is part of thought.
Thus they admit that they're not setting out a refutation (and since when did the most interesting philosophy consist of mere refutation?) but offering a simpler, more fitting concept of mind. This passage also shows that their thesis is an attack on the tradition: even if they achieve it with a "wide computationalism", i.e., a computationalism extended into the agent's environment, this is still revolutionary, because the computational theory has
traditionally been overwhelmingly neurocentric and dependent on an input-output model, with symbolic manipulation going on in between.
Thus while it's true that they seem still wedded to a representational, computational theory of mind, their thesis is at the same time anti-Cartesian, because it helps us get beyond the mind-body, or mind-world distinction, and asserts that what is important in conceiving of the mind is not just what's in the head.