A possible problem with tachyon's code is that it doesn't work unless you happen to be in a zip code with distributors

If you call call the UPSPS National Customer Support Center at (800) 238-3150 you can talk to the them about ordering zip/latitude/longitude data

The documentation for this data is available as a pdf on the usps site.

It seems like most people find a vendor to give them this data.The USPS FAQ talks about it a bit.

I'm guessing that the 'ol a**2 + b**2 =c**2 would be how you would compute the shortest distance between two arbitrary zip codes. No doubt the monk map uses nicer code...

use strict;
use warnings;

my $x1=10;
my $y1=20;
my $x2=-8;
my $y2=2;

my $dist=sqrt(($x1-$x2)**2+($y1-$y2)**2);

print "$dist =distance";
[download]

Of course you have to (pre?) compute the distance between the user's zip code and many of the other zip codes in your database...

--mandog