Food miles: a real issue or a distracter?

The Mercatus Institute has produced a report arguing that food miles – the environmental cost of the distance food travels – is a meaningless concept based on erroneous assumptions, and that the “buy local” movement is focused on the wrong issues.  I don’t know anything about the Mercatus Institute other than what is on its website, and I don’t recognize the names of its members.  Anybody know anything about it?  Here’s what the Wall Street Journal said about this group in 2004.