The great thing about the 'what we need more of..' shirt is that people who don't even know of achewood seem to appreciate it. People have said to me, "that's true!!" and such
I've had that happen, too.
A few years ago on a college campus in California I saw a guy with a "It is impossible to have a good day" shirt (Did I get that right? Since the shirt's no longer available I can't remember). We talked for a minute about Achewood, which he pronounced "H-wood".
That's it, though. I've never had anyone recognize my Science shirt. I've seen a few other webcomic shirts around (Diesel Sweeties, Exploding Dog, what I think was a Penny Arcade garment), but that is all.
Obviously Onstad must be doing well, though. I say this because I can't imagine any scenerio where one's wife had a baby, and she was still cool with her husband spending many hours each day drawing profane cats, if he wasn't making a decent living off of it.