Number 6 in my Top 10 Sustainability related TED Talks, we have a serious and sobering look at what it would actually mean to produce all of our energy from renewable sources by THeoretical Physicist and generally brilliant person David MacKay (asside from physics, he has a Ph.D in Computation and Neural Systems).
MacKay uses some very rough and simple calculations in this presentation to show that there are some very serious constraints to renewable energy technology available today, and even assuming that we can improve the technology significantly some of those constraints will never go away. For example: potential energy per unit area is drastically lower for renewables than nuclear, or coal.
I enjoy MacKay's candid, hard nosed look at what the options really are, and the practical return to what is really important: understanding how we use energy and trying to use less. Only then are renewable energy sources even viable as a replacement for fossil fuels.