modified version of CivicSpace US ZIP Code Database
Sliced and diced version from http://www.boutell.com/zipcodes/
From there
"The database [and README] are copyright 2004 CivicSpace Labs, Inc., and
are published under a Creative Commons Attribution-ShareAlike license,
which requires that all updates must be released under the same license.
See http://creativecommons.org/licenses/by-sa/2.0/ for more details."