Sara at Down to Earth highlights a Cornell study which finds some meat/dairy in a diet is more efficient than a pure vegetarian diet, since it enables use of poorer quality land (study was done in NY and NY has lots of poorer quality land) for grazing.
Very interesting, but I'd like to highlight a different aspect of the study. The idea was to figure out, using a complete diet of only foods that can be grow in NY, the "agricultural land footprint" of the diet. "Locavore" isn't mentioned in the study but that's the definition. They compared 42 diets with different mixes of foods and found that one person takes .44 acre on vegetarian and 2.11 acres eating 3/4 a pound of meat a day. But the best case is NY land can support between 22 and 32 percent of its population. (I'd assume the study used all sorts of assumptions in terms of farming methods and agricultural workforce.)