Speaking for myself, the boundaries just denote the different socio-cultural-economical regions (or subnations, if you prefer) present in the United States. The whole subject is mostly academic in good times, but I think Garreau's boundaries would be pretty good predictors of how the nation would fragment in a general collapse of central authority.
Speaking of which, just to show how things change, the teabaggers' new hero is an ex-KGB agent who predicts the US will soon fragment into 10 separate regions under foreign control:
http://www.alternet.org/story/143816/te ... _next_year
How do you suppose the right wing would have reacted to something like that 20 years ago?