Pete Templin wrote:
Very rusty memory cells on this, but I think the mileage is
0.1 * sqrt ((delta-V)^2 + (delta-H)^2)).
That's assuming same LATA, IIRC.
Close. It's
sqrt ( 0.1 * ((delta-V)^2 + (delta-H)^2)) )
and it doesn't care about LATAs.
It's mostly accurate in the US middle 48 states and
nearby parts of Canada and Mexico.
http://datec.web.att.com/faqs/telecom.htm has more information,
not all of it correct.
Telecordia has pictures
http://www.trainfo.com/products_services/tra/vhpage.html
and will sell you more documentation and software,
though they don't seem to make it easy 
They've also got some material on issues with the
newer coordinate systems such as WGS-84.
Several other people all sell it - most of it's bundled with
databases of NPA-NXX.
http://article.gmane.org/gmane.comp.gis.proj-4.devel/26
points to the free OpenMap software, which has a converter.
http://article.gmane.org/gmane.comp.gis.proj-4.devel/19
has Fortran code!
Google will tell you more.