Cash Crisis for Commuters?

(Post #200) Mark Morrison and others point out that the exurban dream (or nightmare for environmentals) has become noticeably more expensive in recent months, especially in the northern and Atlantic states. Among other possible consequences, they note that exurbian growth has driven the relatively good economy (especially in housing) of the last several years. Carpooling, less comfortable home temperature settings and smaller cars may help, but the exurbs eventually will need employers to continue to thrive.

This is a topic I have rarely seen discussed on personal finance blogs. At what point does moving to the country (if you work in a big city) become the equivalent of supercable or a gas hog SUV from a frugality standpoint?