How many people are needed to maintain current living standards?

Andrew West, citing Charlie Stross, sends along a question:

What is the minimum number of people you need in order to maintain (not necessarily to extend) our current level of technological civilization?

I'll treat this as a steady-state question and not commit to any particular time frame.  Stross wrote:

I'd put an upper bound of about one billion on the range, because that encompasses basically the entire population of NAFTA and the EU, with Japan, Taiwan, and the industrial enterprise zones of China thrown in for good measure. (While China is significant, more than half of its population is still agrarian, hence not providing inputs to this system).

I'd put a lower bound of 100 million on the range, too. The specialities required for a civil aviation sector alone may well run to half a million people; let's not underestimate the needs of raw material extraction and processing (from crude oil to yttrium and lanthanum), of a higher education/research sector to keep training the people we need in order to replenish small pools of working expertise, and so on. Hypothetically, we may only need 500 people in one particular niche, but that means training 20 of them a year to keep the pool going, plus future trainers, and an allowance for wastage and drop-outs by people who made a bad career choice.

My casual, seat-of-the-pants estimate is that a world of one billion people — mostly from the wealthier countries — would be about fifteen percent poorer for those people than today's world.  Even very poor countries often supply valuable commodities or raw materials, in addition to buying some exports and expanding the scope of both comparataive advantage and increasing returns.  What's the implicit assumption about settling those countries and reaping those resources?  Are we dealing with a Nigerian government backed by five million very poor Nigerians?  Or is Nigeria an empty land, open to settlement as a southern colony of Switzerland?  I am assuming the former and that means those countries have much smaller economies, and higher fixed cost problems, than today.

Under this scenario the United States loses much of its Asian immigration and Arab immigration, two successes even if you disagree with me about the successes of Latino immigration.

Alternatively, the thought experiment could be shrinking the world's population but keeping the rich-poor proportion constant in the population.  In that case the number of people needed to maintain current standards of living, or even fifteen percent less, is much higher.