The government has decided that the "border" extends 100 miles inland from official land/sea borders. This apparently covers 2/3rds to 4/5ths of the U.S. population.

Coverage on the 100-mile-thick ACLU-dubbed "Constitution-Free Zone":

Wired: http://www.wired.com/threatlevel/2008/10/aclu-assails-10/

Ars Technica: http://arstechnica.com/security/news/2008/10/aclu-23-of-us-p...

(Note the entire land area of the Hawaiian islands are covered, as is the entire state of Florida.)

I think it's probably even worse than that, though, because International airports count as "borders" no matter how far inland. So I would not be surprised to see this extended to a 100-mile radius of all international airports as well, which would cover almost everybody.