Seeing Keith Ray's post on ethanol I remembered reading various second-hand accounts about the net energy output of ethanol. After a bit of Googling, I thought I'd post the relevant links:

• Ethanol -- Just Another Dirty Fuel lists several links on the subject. Particularly...
• Ethanol Fuels: Energy Balance, Economics, and Environmental Impacts are Negative by Dr. David Pimentel. The conclusion of this paper is that ethanol production produces a 29% negative energy balance. This seems to be the most cited anti-ethanol work.
• Wikipedia: Alcohol as a fuel discusses both sides of the debate, though it seems to lack reference to primary sources.

It's complicated how exactly you should judge an energy source, but from what I can tell it's mostly a question of how bad ethanol is.

One argument is that at least you can use domestic non-renewable energy sources to manufacture ethanol, like coal (which itself cannot be used in cars). But the environmental impact (especially considering greenhouse gasses) seems atrocious, any way you cut it. I can see why there's some argument for ethanol based on energy independence, but I can't see any environmental justification. But biomass proponents have always confused me. I think biomass seems naturally wholesome, since farmers and corn and wood and peat are all very wholesome seeming. Still, ADM is about as un-wholesome a company as you can get, and that's where ethanol is really coming from.

Apparently ethanol is big in Brazil. I don't know exactly why (is sugar cane more efficient than corn?) -- but I suspect for the same reasons as in the US, that it's just a complex and wasteful way of subsidizing farming.