Well, overpopulation is a potential problem if things keep going a certain way. The shrinking populations of the developed world still consume far more resources and contribute far more to environmental destruction than everyone else. If all of those billions of people living in the third world were ever to start living like us AND their populations continued to grow at the same rate that they are now, then shit would hit the fan. However, neither of those really seem to be an issue, because developing countries aren't going to reach Western living standards any time soon, and in all likelihood their birthrates will have dropped to manageable levels well before that.