Me and my pesky, middle of the road opinions...
First off, yes, XKCD hits the nail on the head. Idiocracy is a comedy. It uses broad strokes to exaggerate concepts into parodies. There is no actual, strong device by with stupidity is selected FOR. Those that you do see are canceled out by also weak selections against stupidity. Overall, as XKCD mentions, were likely to stay just as smart as we are.
This, however, is not entirely good.
Humans live in an increasingly complex world, and the problems we are addressing require more and more intelligence. There is less and less emphasis on fact remembering, since it is easier and easier to externalize knowledge, so more and more we need to be capable of useing information we access in intelligent ways. As technology advances, the Human brain seems to be at a standstill. Sure, we aren't actually going backwards, but are we going forwards? I don't think there's pressure to do so. Wouldn't it be an imperative to try to slowly evolve into something better, as opposed to standing still?
I'm not going to be an advocate of sterilization programs, or other heavy handed eugenic programs. I don't think they are necessary, or even realistic to undertake. But I do think that as a society, we need to realize where were going, and we need to start valuing intelligence more. We need to hold a research scientist on a higher pedestal then a sports superstar or a celebrity, and intelligence in both men and women needs to be seen as attractive. That sounds like a better world, even without the natural selection part.
Now if only there were some cultural movement gaining popularity with the opportunity for social interaction and networking, that valued the participants capacity for such actions as lateral thinking, puzzle solving, and pattern recognition. You know, some sort of institution where people engage in activities that are fun and engaging, yet also favor a capacity to understand and interact with a rules system...what could that be...