Harvard environmental science professor Michael B. McElroy takes a sober look at ethanol:

The balance in terms of emission of greenhouse gases is close to a wash for the United States: the reduction in net emissions of carbon dioxide obtained by using corn rather than petroleum as a “feedstock” for motor fuel is largely offset by additional emissions of the several hundredfold more potent greenhouse gas, nitrous oxide, formed as a byproduct of the nitrogen fertilizer used to grow the corn.

I don’t know anything about McElroy’s potential biases, but he certainly comes off as a serious-minded critic rather than a hit man. So if you’re interested in a hype-free view of ethanol production in the U.S., this seems like a good place to start.