The urban milestone

Recent studies announce that by 2008 half of the world's population will live in cities:
Urban population to double - UNFPA
Of this, however, 80% will be in the developing world by 2030, with as much as 2 Billion slum dwellers.
So when we see these numbers the image to have is not of more and taller megalopolis skyscrapers but larger and larger urban sprawls, with conditions way below what we westerners would consider a bare minimum standard of living.
It is hard, when seeing the numbers skyrocket like this, to not seriously consider the Malthusian scenario - that some say it is already starting to happen in places like Africa.
And again, taking this bit of info over our "inner fences" and factoring in sea level rises due to global warming and the fact the vast amounts of the world's population live in areas that could be seriously affected (see Bangladesh) we see that the urban shift is perhaps but a minor dislocation compared to the migrations that could be caused by these more drastic changes.
The authors of the studies pointed out that more careful planning is required to properly address this urban milestone (which is probably an understatement, since most of these up-and-coming megacities have had minimal, if any planning - and have grown organically over the years).
Another consideration to be kept in mind is that the slums offer the ideal conditions for the onset of pandemics - with the lack of hygiene and close human contact that can promote a rapid spread.
On a more theoretical level the predominance of urban slum population will most likely cause a growing imbalance in resource allocation (food, water, jobs, living spaces) that can at some point determine a conflagration to restore balance. For that matter, no one is safe, even us in our glass and concrete-clad "caves of steel" of the "civilized world".