Well, total fertility rates (TFR--more indicative than just plain birth rates) ARE going down in a lot of places, since the end of the last century... not just the West, either. Take a look at Thailand, for a great example of a country going through the demographic transition very quickly (demographic transition means the change from high birth and death rates, to low birth and death rates)... in the 1960's, the TFR was around 6, and today it's significantly below-replacement at 1.64. I think that cultures can and will change, with time and education (especially for women, regarding birth control), but a LOT of time will have to pass for some cultures. I'm optimistic, though.
It would also help (as Baraka said) if we all became vegetarians and ate locally... but I'm not ready to take either of those steps yet. Try eating only local vegetables in ICELAND!!!
Yeah, that would control the population real fast... we'd all be dead, here. It's happened a few times in Iceland's history, up until the 1940s when they actually became prosperous (as a result of the American occupation during WWII, in fact--they hate that).