I don't believe mental events have nonphysical causes, which is precisely why I consider zombies to entail causeless effects.

And yeah, perhaps I'm just reading too much into "metaphysically impossible" as distinct from "impossible"; the truth is I really don't know how to think cogently about what would be impossible if the laws of nature were different.

The "we're all zombies" bit was intended with tongue in cheek; it's actually a direct quote of his from some book or another -- Consciousness Explained, probably -- that I read like 20 years ago and stuck with me. He isn't entirely serious when he says it, of course, and IIRC has a little footnote that says "To quote this phrase out of context would be the height of intellectual dishonesty." or words to that effect.

EDIT: That said, another relevant Dennetism that stuck with me was his response to an undergrad at a seminar I was listening in on years back. The undergrad said, in effect, "But I don't feel like a merely computational process!" and he replied "How do you know? Maybe this is exactly what a merely computational process feels like!"