This reminds me of a talk by economist Robin Hanson: http://www.vimeo.com/9508131

Although his presentation is pretty futuristic, a good chunk of it focuses on the economics of machines replacing people in tasks.

His argument goes like this: There's a range of tasks that need to be done in an economy. Some of them are more efficiently done by machines and some by humans. Together, all these tasks grow the economy, and over time machines have been able to do a greater portion of these tasks efficiently. Also, machines can be mass-produced, while it takes longer to scale and train people. While the growth of machines has augmented humans for most of history, it's certainly possible for machines to replace humans in many tasks, such that the per-capita income of humans goes down even as the economy grows.