It really comes down to the technology that allows us to survive - food creation, water acquisition and shelter. We've made great strides in each area which is allowing us to grow beyond what we thought was capacity.
I think that the reasonable cap would be around 11-12 billion, given the kinds of technological advances we've seen. Do I advocate getting that large? No. We're too populated (in terms of what we need) as it is. Fortunately, the growth of population is slowly falling, which is a good sign.
If you wanted to control the population, you would stop sending so much food aid to Africa. The rest of the world would take care of itself. Also, if America and other countries could remove themselves from dependancy on mideast oil, it would stymie their growth as well, as the middle east and Africa account for the vast majority of population increases:
Back from the dead, I'm afraid.
