OK, so maybe that's not more surprising, but bear with us, because it doesn't stop there: When you see someone cruising around without a shirt, it's not just their IQ that you think is lacking -- you suddenly start to assume that they suck at other stuff, too. In one study, both men and women were shown pictures of "Erin" or "Aaron" in different stages of undress but with the exact same biography. The more skin the person in the photograph was showing, the more the test subjects assumed that Erin/Aaron was not only a blundering dumbass, but morally incompetent as well.
In an interesting variation on the experiment, researchers wired some dude up and gave the test subjects the ability to administer an electric shock (because it's not really science if no one's screaming). You probably think that people would be more likely to shock the guy if he was shirtless -- because, hey, he's probably too dumb to hold a grudge -- but the results were actually the exact opposite: People were more likely to light him up when he was fully clothed than when he was sans shirt. So apparently, although we tend to assume that scantily dressed people are stupid, we also see them as more likeable and/or deserving of our protection.
Either that or they were just pissed at being stuck in the "clothed people" group.
So to summarize, people will think you're stupid if you sexy yourself up in an attempt to increase your chances of getting laid, and you in turn will think people are stupid if they're trying to call sexual attention to themselves.