The law of unintended consequences. Something always happens you didn't anticipate, but you really should've had you done any thinking beyond the obvious benefits to you.

Case in point, the cotton gin. Cut down on the number of people needed to de-seed cotton bolls. Thus it made providing cotton for textiles easier and cheaper, which made it profitable to provide cotton in bulk. But, you still had to harvest cotton by hand. With larger cotton fields, and more cotton fields, the demand for slaves increased, which, in turn, meant an expanded slave trade and so more crowded conditions on slave ships and more slave ships overall. All because of one mechanical device.

Why not just switch from petoleum back to whale oil? Its a tried and proven technology, and whales are renewable aren't they?

It turns out that in producing biofuel (either alcohol or diesel), corn and other cultivated crops (except sugar cane) are not as good as untended prairie.

Cultivated crops require more fossil fuel to grow than they produce. But when you don't cultivate the crop, and just let it grow, weeds and all, two nice things happen.

First, you don't have to buy pesticide, fertilizer, or seeds, since the stuff just grows. Costs plummet.

Secondly, it turns out that the more types of plants you allow to grow, the more overall mass per acre that grows. It's this little thing called diversity, which is overrated in committees but works really, really well for growing biofuel, 3 times better than a monoculture.

"...could increase the price of agricultural commodities with negative economic and social impacts, especially for the world's poor who spend a large proportion of income on food."

Could? It already has. Corn doubling in price has almost no effect on us here in the US,(so what if the grocery no longer puts corn on special at 3 ears for a dollar,we can still afford 2 for $3)but in Mexico it means they are having a shortage of cornmeal which is the staple for their poor. Those folks may not be able to eat at all.... So even more of them will be heading north!

Also I read somewhere recently that if ALL of the arable land in the US was used for biofuels it would only produce 18% of the fuel we now use.

Obviously we need a better answer.

I did a full post on this.

But an unhealthy (but highly diverse) coalition of farmers wanting grain subsidies, oil companies, ag companies, and environmentalists afraid of both nuclear power and oil spills has kept us from being weaned of foreign oil.

I think I'll look into the numbers on this. People claim that there isn't enough land in the country to grow the amount of biomass we would need to convert to biofuel. I wonder.