How far is local?

In discussions of the relative merits of local vs. long-distance food supply 
(here and elsewhere), the term "local" is usually left somewhat vague. How 
far is "local"?

To take a particular case, from the CSAs that any of you are familiar with,  
from how far do members actually come to pick up their weekly share? And up 
to how far should the CSA draw its members from while still being considered 
a local food system?  

William Lockeretz
Tufts University