Among the pseudo-hippie university students I am currently surrounded with, its quite trendy to profess allegiance to eating only locally grown food. The idea, I think, is that this is supposed to be good for the environment - and your local farmer (it seems like the support you are obligated to give a farmer decreases proportionately to the distance between you and the farmer). Not surprisingly, like most ideas conceived by environmentalists with little understanding of modern technology, it ends up making little sense in terms of the actual environmental impact. The Boston Globe ran an article about this today:

...a gathering body of evidence suggests that local food can sometimes consume more energy -- and produce more greenhouse gases -- than food imported from great distances. Moving food by train or ship is quite efficient, pound for pound, and transportation can often be a relatively small part of the total energy "footprint" of food compared with growing, packaging, or, for that matter, cooking it...

Judged by unit of weight, ship and rail transport in particular are highly energy efficient. Financial considerations force shippers to pack as much as they can into their cargo containers, whether they're being carried by ship, rail, or truck, and to ensure that they rarely make a return trip empty. And because of their size, container ships and trains enjoy impressive economies of scale...

"Local food systems are often built around small-scale logistics," says Chris Foster, a research fellow at England's Manchester Business School and co-author of a December 2006 study on the environmental impacts of food production and consumption commissioned for Britain's Department for Environment, Food and Rural Affairs. "You begin to make more trips in cars. More food is shifted around in small trucks and vans, which are relatively energy-inefficient ways of moving."

The difference can be dramatic, according to Rich Pirog, a food-systems researcher at Iowa State University's Leopold Center for Sustainable Agriculture. A bag of potatoes shipped from Idaho to Boston by rail, he estimates, is likely to require less energy in transit than the same bag of potatoes driven from Maine to Boston in a farmer's truck.