This is great, obviously, but overpopulation is a huge problem and regrettably that problem will now get worse. The predictions for the maximum population expected at mid-century have recently been adjusted upwards. Resources will be depleted ever more quickly if poor people get as rich as Bill predicts.
I have no answer. Surely we cannot withhold medecine from poor people to prevent overpopulation. But two decades from now overpopulation will be the major worry on everybody's mind if this trend continues.
Edit: The assumption of negative growth once countries hit a certain level of development/stability/richness has recently stopped being true [1], I probably should have mentioned that in my first comment
1. http://en.wikipedia.org/wiki/Projections_of_population_growt...