With guidance from the great Jeff Larson of ProPublica, these bounding boxes were extracted from the U.S. Census Bureau's 2010 set of shapefiles for American counties.

They were processed using Postgres' GIS, aka PostGIS. 

On the import, you need to specify LATIN1 as the language.

This code kicked out something that could be exported as a CSV:
drop table if exists exportthis;
create table exportthis as
select
statefp10,
countyfp10,
countyns10,
geoid10,
name10,
namelsad10,
st_astext(st_expand(the_geom,0)) as boundingbox
from tl_2010_us_county10
order by statefp10, countyfp10;

The bounding.py file here did some more processing to turn the bounding box into something more easily used and parsed.