I worry about lots of stuff; and the idea that we are now at 6.7 BILLION people on the planet kinda terrifies me. Of course, when I mention this to people, I get funny looks (at best).
Although we Americans live in the "developed world," overpopulation does affect us. It can be hard to see, which is why in the comic I tried to point out some of the everyday consequences of too many people; I didn't include some of the more disturbing trends like climate change and peak oil that no one wants to discuss.
Now you could argue that many of our problems are the result of poor resource management---and you'd be right. But population and limited resources are two sides of the same coin: there are too many people using too much stuff and doing it incredibly inefficiently (and unfairly). And even if we were to become extremely conservative*, we're still going to have tremendous challenges in meeting the needs of the 9 billion that are expected to be here soon.
*Obviously, I'm using the word in the "using less" sense, not in the "let's give tax cuts to rich people" sense.
Excellent short talk on population growth from TED.