OK, let me have a try at the population issue, based on a proposal I wrote for the CGIAR in the 1990s.
The reason no politician or leader talks about reducing the population is that it is political suicide to do so. Note how Mr Costello and others promote more childen, not less. This is based on their reading of the mood of the 'average' citizen.
There is only one thing that successfully, universally and voluntarily reduces the birthrate: prosperity. To reduce the number of births you need to lift the average income of poor people above $US2000/year. Countries which have succeeded in this almost invariably experience large decline in the number of children/mother. This implies stimulating economic growth in poor countries as humanity's chief contraceptive.
Urbanisation also lowers the birth rate as children have a lower economic value in urban than in rural society. However I'm not sure that urbanisation is a very sustainable solution to overpopulation, given its other impacts, especially in demand for water, energy etc.
If we aim to lower the world population to 1.5-2 billion by 2100 voluntarily, we are going to have to bring the poor out of poverty very quickly, and this implies a mid-century 'hump' in demand for the earth's resources that may or may not be sustainable, but either way will be temporary. It's what I've been writing about in the context of food etc.
How do you get this on the political agenda, not to mention the religious agenda? - well, it is very hard, as everyone here seems to acknowledge. But it can be done by advancing rational and humane ways to do it, like this, in public fora like this. It won't happen if we just sit round and grouse. We need to generate global public awareness in the same way the IPCC has done for climate change.

Julian, you have said: “Note how Mr Costello and others promote more childen, not less. This is based on their reading of the mood of the 'average' citizen.”
Utter balderdash!
It is, instead, based on the antediluvian state of their intelligence.
Australian women with a fertility rate currently at 1.8 give evidence as to “average mood”.
The Australian Parliamentary Group on Population Development are also aware of that.
We do have global public awareness. Have had it, and recommended direction, since 1994 in Cairo, as you no doubt know.
The awareness is pervasive. Action upon it has been actively prevented by politicians leaned upon by fundamentalist pressure groups of either religious or voodoo-economics origin.
It doesn’t take much intellectual effort to realize that lifting the average income of poor people above $US2000/yr is impossible while their fertility rates remain above 2.1. All the good work you promote will go down the drain if adequate efforts aren’t made to facilitate that. While you remain silent about it in the mainstream of your public utterances, you remain part of the problem – facilitating continued expansion of ever-desperate consumers. You’ve got a lot to contribute – get to it.

JulianC,

I liked your article. It complements what the Earth Policy Institute has to say about global grain supplies:

http://www.earth-policy.org/Indicators/Grain/2006.htm

It is true that prosperity eventually leads to lower birthrates, although there is often a considerable lag. (Remember the very high birthrates after WWII.) Mexican immigrants to the US have a considerably higher fertility rate (on the order of one child more per woman) than comparable people back in Mexico. The ones in the US believe that they can now afford the very large family sizes that are idealised by their culture. The second generation has fewer babies, but still a lot more than the US average. Even in prosperous societies with low birthrates, governments can still promote population growth with mass migration, even though ordinary people are less than keen on it all over the world:

http://pewglobal.org/reports/display.php?ReportID=258

The problem is not the attitude of ordinary people, but of elites who want a cheap, compliant work force, effortless profits from land speculation, savings on training costs, undermining of support for the welfare state, etc., etc. Because of the additional consumption, an extra person in a developed country is a far greater disaster than an extra person in the Third World.

In any case, all the calculations indicate that it would take the resources of three Earths to give all 6.75 billion of us a Western European standard of living. (See the Redefining Progress site and a short article by Daniele Fanelli in the Oct. 6, 2007 New Scientist, p. 10). I can't see any alternatives other than Chinese style authoritarianism at best and a Rwanda style solution at worst.

Julian, I think you overestimate the role of money and underestimate
the role of religion, when it comes to the population debate.

I've spent alot of time investigating this one and everywhere it
seems similar. Yes there are women in the third world who simply
cannot afford family planning measures, they are too poor. So
hundreds of millions of women use none. No wonder there is a rising
population. Yet all surveys I've seen show that these women would
use family planning, if it was available.

What also becomes clear, is the overwhelming influence of the Catholic
Church in the third world. There are plenty of women in
places like Nicaragua, the Philipines etc, who would love to
have their tubes tied, but religious influence on politics, prevents
this happening. The Catholics are determined that the Muslims
will not outbreed them and there is no better global lobby group,
then the Vatican.

As we see even in Australia, the Church has an influence on those
politicians determined to get their Catholic ticket to heaven.