Overpopulation due to technology will probably get us. By this, I mean nanotechnology (or even smaller in the future) and the ability to artificially produce white blood cells and tons of other different cells. If WBC's are continually available and there is never a shortage, people could begin living 400-500 years longer. I have even heard that the life expectancy could be live 900 in the next 60 years or so.
http://www.nanotech-now.com/Art_Gallery/tim-fonseca.htm
Of course, this tech is still being worked on and isn't really a reality yet, but I could see it becoming huge eventually....maybe the first step into seemlessly integrating humans with computers? Who knows.