It's time to move on to the evils of Newt Gingrich and other fertile fields, but for those of you who have inquired about the basis of the numbers in my locavore story, here's a quick guide to the sources.
First, as to the assertion I am attacking a "straw man," here and here are some of Bill McKibben's typical pronouncements on the subject, which were in fact why I began looking into the issue at all. It just didn't sound right to me that transportation could be a significant fraction of the energy costs of food production. It turned out I was right.
For the energy efficiency of long-distance transportation, I used (in very round numbers) about 400 ton-miles per gallon of diesel fuel for trains, 150 for tractor-trailers. Here is one source; you can find many other recent estimates that are in good agreement with these numbers. Note that fuel efficiency of tractor-trailers has more than doubled since the 1980s. The key point is that even if these approximations are off by a factor of, say, two, it doesn't change the conclusion: the energy required to ship produce across the country approaches the trivial in the overall picture.
For the figure of 5,000 kcal of total fossil fuel inputs to get one lettuce to your dinner table, see Matt Ridley's The Rational Optimist, p.150. NOTE ADDED 8/23: You can find other estimates ranging from about 2,000 kcal to 8,000 kcal. David Pimentel's 1980 calculations of tractor-trailer fuel consumption that many have cited are completely out of date given huge gains in fuel efficiency since then, and were questionable even at the time. Refrigerated tractor-trailers get about 20% lower fuel economy than non-refrigerated ones; long-haul refrigerated tractor-trailers get about 6 miles per gallon, which agrees well with the 100–150 ton-mile per gallon figure I used in my original article. A more recent study by Pimentel (pdf here) arrives at total on-farm fossil inputs of about 3 million kcal per acre for corn, the bulk of which is the energy cost of nitrogen fertilizer (about 1 million kcal) and machinery (about 0.5 million). Many vegetable crops use even more N per acre than does field corn (e.g., 150-200 lbs for watermelons) and are easily as machinery-intensive if not more. At 20,000 heads of lettuce per acre, it comes down to about 150 kcal per lettuce for on-farm production; 400 kcal for hauling 3,000 miles (100 kcal if by rail); and the rest is home preparation, local transportation (a huge input), and home storage (ditto). I just looked in my refrigerator and estimate I have 25 pounds of food in there at any given time, which is probably also about the net weight of groceries I bring home from the store each week. That means each 1-pound head of lettuce racks up nearly 1,000 kcal just for its share of my once a week trip to the store and running the refrigerator for that time. But let's not get lost down the rabbit hole: the whole point is the big picture, namely that transportation is 14% of the energy inputs of the whole U.S. food system, and only a minor part of that 14% is long-distance transport. You will easily blow the entire energy savings of "buying local" with one trip to the farmer's market in your car.
The proportion of total U.S. energy consumption that goes to residential use, the food system overall, and various components of the food system (transportation, on-farm production, etc.) comes from the U.S. Energy Information Administration here and here and the pdf fact sheet from the University of Michigan here. (Note that total U.S. energy use is almost exactly 100 quads a year; on-farm inputs are 2 quads per the University of Michigan fact sheet.)
Data on land in U.S. farms comes from the U.S. Census of Agriculture and the Historical Statistics of the United States.
Thanks to hybrid varieties and the extensive use of nitrogen fertilizer (which is made by using methane as a hydrogen feedstock and energy source but which, yes, derives the nitrogen directly from infinitely renewable nitrogen in the air), yield per acre of grains has tripled or more in the last century. That translates to hundreds of millions of acres of land that has been spared from the plow.