Here's what Matt had to say:
I'm honestly not sure about the enforceability of these sorts of disclaimers, and searches of Lexis and Westlaw didn't turn up anything useful. I expect that they're generally seen as enforceable to a certain extent, however from my first-year law student point of view I'd say that they're probably not the cure-all companies think they are. After all, by the time you get to the disclaimer, you've already read (and possibly copied/forwarded/etc) the email.
He's going to poke around a little to see what he can learn, but he's got a good point. Sheesh. Fingers crossed that they don't move disclaimers like this to the top of our emails. Ha!