Thanks for all the replies. As I said in the post, I also don't think Adams is completely serious. Here is the weaker version of his argument that I find interesting: if someone can make you (or maybe other rational/informed people) laugh at your beliefs, should that cause you to reassess your level of certainty in those beliefs?
In other words, I don't think Adams really believes that someone "successfully" mocking your opinions automatically makes them false -- but he's asserting at least some connection between this kind of humor and truth. Which feels right to me, though I can't really articulate it any better than he did.
Or maybe it's more of a connection to self-deception -- the easier it is to laugh at your own beliefs, the more likely they are to be somehow insincere, regardless of their truth or falsehood.
If it's a belief you've previously thought of as obvious and left unexamined, then this is probably a useful heuristic. Otherwise, no.
I'm not sure what the protocol is for linking to or quoting another site on LW, but I thought this would fit here for two reasons: first, because I'm curious what people here think about his 'mockability' test, which seems to be half in jest but (I think) has a serious point at its core -- and second, because I think there might be people here who want to take him up on his challenge.
(Obviously I am not Scott Adams, and I have no connection to him nor any reason to promote his blog.)
http://dilbert.com/blog/entry/mockability_test/