Here's an interesting piece by Harvard's Martin Feldstein, who was Council of Economic Advisers head during the Reagan administration and is a pretty good economist. He argues that because the expected future price of oil causes those with supplies to keep it in the ground rather than sell it -- because they expect the price to be even higher in the future -- anything done now to reduce the expected future price of oil will have an immediate impact on the current price of oil. He notes that steps to reduce demand -- higher CAFE standards, for example --would help, as would steps expected to increase supply, even if the expected new supply wouldn't actually come online for a while. Thus, I infer (he doesn't say it directly) that just announcing that we're going to start drilling offshore or in ANWR should have a downward impact on the price of oil now.
I find the argument quite persuasive. The same argument applies to food. Announcing that the ethanol program would be ended or suspended for a few years should place immediate downward pressure on the price of corn. It might well end up higher than a year or so ago -- increased demand due to increased affluence in China, India and the Gulf states would still be a factor, as would the drought in Australia that to my knowledge hasn't ended yet -- but it still should go down noticeably. The greatest impact would be on those on the edge of dire poverty in underdeveloped countries.