A calculation of the greenhouse gas burden for meat production is at Newscientist (18 JULY 2007).
This calculation has little direct relationship to the increases in the price of food.
There is pretty general agreement that the diversion of massive amounts of corn to ethanol production has driven up the price of corn. This has had an immediate and direct impact on the price of tortillas in poor districts in Mexico.
Diversion to corn production of fields previously used for other crops has resulted in lessened availability, and hence higher prices, for a number of other crops. Soy bean production in the US was down quite significantly last year. (The impact of diversions to corn on silage crops (alternative cattle feeds) has not been reported where I've seen easily cited numbers, but has apparently been "noticed.")
Increases in the price of corn have led everyone from livestock feeders to pet food manufacturers to look for other substitute grains, which of course has driven the price of other grains upward.
Widespread crop failures, partly due to localized drought, and partly from rice-virus "epidemics" in a couple of major rice producing/exporting countries, have resulted in very real shortages - regardless of price. Of course, when there's an actual shortage, the price goes up so that the well-off part of the population can get what they want, by outbidding the poor.
US Ag Dept estimates for the crop season just now begining are optimistic about a slight reduction in corn planting, with some re-diversion of land back to other food/feed crops; but at present it's just an estimate.
The increase in corn-based food costs is most critical in Mexico and other places where corn is the dominent staple food. Increases in rice price, and shortages at any price, likely affect greater numbers of people in SE Asia. It's difficult to say, from published reports, where the likelihood of (life-threatening) malnutrition is currently most critical; but many of the "humanitarian" agencies are reporting that price increases already have brought them to zero-fund status, unable to purchase food stocks to distribute.
Isolated reports (in media I was seeing) began been popping up about a year ago, ramping up pretty linearly as time goes by. Reports describing shortages as "critical" are perhaps 4 to 6 months old now. (There is no indication that the number of people having difficulty buying the food they need has reached a peak.)