Are you guys wanting to build Coruscant or the Megacities or something here?

Civilisation starts in the fields not the cities: check out all the early civilisations in the river basins etc, why? Because they got their agriculture up to the level they could support large non-food producing populations in these densely packed urban environments.

The problem isn't going to be feeding all these people or anything, we can do that (they won't be getting McDonalds everyday thats all) - its the fact that most will be Chinese, Indian, SE Asian or African - all areas with massive poverty already. The rich areas will not have these problems. This is going to exacerbate those poor countries problems, and strain the ability of the local environments to cope - it does Sweet FA good if the USA and Europe's ecology is fine if the rest is screwed, as will happen. The huge populations will never be able to get a large middle class and eliminate poverty - there will be just to many people to get the economg going like that (what the hell are they going to do for jobs? 3 billions more people, most with low education and in deprived areas?). The Chinese will maybe climb out of this in time with the one child policy saving them some of the worse, but the rest of the third world has big problems coming from this.

The answer of course is to colonise space ASAP, but we screwed that one up...