My personal opinion is that we should hold off on a second airport. My reasons:
The price of oil, despite its current low, is set to rise and rise as oil becomes more and more scarce.
Sydney airport is operating at 84% capacity and is as I understand it requiring an extra 2% per year at the current rate of city growth/ travel demand. This gives another 8 years at the current growth rate before the airport reaches total capacity.
As far as the best studies note, we are either at peak oil, or have just passed peak oil. Very soon we will see production drop. The last time oil production dropped whilst demand was maintained, oil went up 400% (the decrease in production was only 5%).
At best case I would say that oil production will be forced to drop by at least 5% by 2015. The increased demand from developing nations for oil may quicken this figure. This means that we will see a massive increase in the price of jet fuel before we see Sydney airport reach 100% capacity.
This increase in jet fuel could well more than double the price of air travel, by simple supply and demand forcing travellers to limit overseas jet travel.
Hence, we wont need a second airport, because in the future we simply wont be travelling as much.
We all have visions of the future where we all zip around in jet cars but at this stage it simply isn't going to happen. At the moment the average person consumes the same amount of the worlds resources as a 35 tonne elephant would. The world simply cannot meet the energy needs of 6 billion 35 tonne elephants.
I know a lot of people will think that technology will save us. However, there simply is no technology on the horizon or otherwise that can replace the energy density/weight of jet fuel. Batteries in planes won't get off the ground, and methanol based fuels etc are made by felling the last of the worlds rainforests to grow palm oil. It simply wont work.
We better get good at cycling, because that my friends is our future....