What is the carrying capacity of the Earth?

The theory that the Earth can't support an ever increasing population indefinitely was first proposed in 1798 by Thomas Malthus. Scientists have dismissed this theory until recently, saying technological advancement would allow resource production to keep up with population growth. In today's energy-conscience society, scientists are beginning to reconsider their dismissal. Estimates on Earth's carrying capacity range anywhere between 2 billion and 40 billion. If the average middle-class American lifestyle was enjoyed by everyone on the planet, estimated at 3.3 times the necessary sustinace level, Earth's capacity would be about 2 billion. If, however, everyone only consumed what they needed, a limit of 40 billion seems reasonable.
The worries about the capacity of Earth come because advancing technology is not helping as predicted. This is because instead of using technology to do more things with less resources, therefore increasing the carrying capacity, technology is being used to further the way things are already being done. Consider energy production as an example. Instead of switching to things like solar energy, whose biggest faults are only money, technology is being used to find ways to extract more fossil fuels, which are non-renewable.