NOPE. Sorry everyone, David Pogue has published an article as a follow up to his review which is basically a Q&A about the thing, and he specifically says that the landscape mode works only during web browsing mode, not for any of the messaging applications.
That said, he also prefaces the article by saying that the secret answer to many questions he answers "no" is actually "coming soon via software update" but of course there's no way of knowing if he's talking about this in particular.
My guess is that Apple will watch and listen. SJ probably really hopes that everyone will "see the light" in the portrait mode keyboard, and learn to live with it (if not actually love it). But if the clamor is there, which I predict, then they'll enable it...
TM