My impression is that a big part of the price increase in food has been the rise of the middle class in China and India. They want (as most people everywhere do) to eat more and better, and often to eat more meat. Meat is much less efficient in terms of productivity than grain is, so an increase in meat production to meet this demand translates into less grain for direct human consumption.
Also, the price rise in oil (which is partly but definitely not totally due to the same up-and-coming middle class) has dramatic effects on food production costs.
Remember that a significant fraction of the corn that is converted to ethanol in the US is from grades of corn that would not be eaten by people, and that much of the cane in Brazil that is used for ethanol production is waste material that wouldn't be eaten by people. So it isn't a one-for-one switch of food-producing land to "gas"-producing land.
