I'm going to throw a book recommendation out there for anyone interested. It's called the Collapse of Complex Societies by Joseph Tainter, and he has a very interesting/thought provoking hypothesis.
http://www.amazon.com/Collapse-Compl.../dp/052138673X
Also a great quote as well.
"Anyone who believes exponential growth can go on forever in a finite world is either a madman or an economist."