Ezra does an able job of exploring some of the problems with conflating the local food movement with environmental goals. Basically, focusing on how far food travels is a lousy way to cut carbon and not particularly relevant to improving the healthfulness.

Interest in local, fresh food started among people who were mainly interested in quality. But somewhere along the way, eating food that was produced near you  became an end in and of itself, a solution in search of a problem. It ended up that way in part because local is a powerful heuristic -- a rule of thumb that steers you right more often enough when making decisions in our current food marketplace. And as Ezra writes in another post, our current food system massively subsidizes all kinds of unhealthy and environmentally destructive things. But, since local and regional food systems seem like a fairly appealing idea, they soon started to get touted as the goal, not the solution to some other set of problems. Thus, Michael Pollan's recent New York Times manifesto has plenty of good suggestions for removing perverse incentives in our food policy (we'd be much better in his ideal world than our current one), but it also seems to take for granted that there's a need for "reconnecting the American people with the American land" or for "stewardship of the land ... self-reliance and ... making the most of local sunlight to feed one’s family and community." All of those things seem dandy to me, but I just don't seem them necessary as public policy goals. 

And once you've fixed on one solution, and started advocating it, the solution itself stars to seem inherently good rather than just a good way to fix policy problems. This is by no means unique to food; it's a common problem in policy making. Once you settle on one policy fix it's simple human nature to see it as a great solution to all kinds of problem -- when you have a chef's knife, everything looks like it needs to be diced. And that leads to deeply silly arguments like this from Pollan's manifesto:

For nations that lose the ability to substantially feed themselves will find themselves as gravely compromised in their international dealings as nations that depend on foreign sources of oil presently do. But while there are alternatives to oil, there are no alternatives to food.

Countries that don't produce their own oil have foreign policy problems because oil is produced by a very small number of countries. Most countries in the world have the capacity to produce significant amounts of food. Moreover, oil production is tightly controlled by the state in these countries and so can be varied as an instrument of policy. We feel to keep supporting the government of Saudi Arabia because oil prices could skyrocket if the country wanted them too -- it's hard to see any country exerting that kind of control over the food supply. I suppose countries that don't like us could stop shipping us food, but they could just as easily deny us medicines, finished goods, or any one of the many things that make modern society workable. Interconnectedness is an inherent part of our modern economy and there's simply nothing we can do about it. Importing food makes the nation vulnerable to changes in price, but local production makes people much more vulnerable to climate. Self-sufficiency as a nation is not only unachievable -- it's not even desirable.