I can't think of a better approach.  The assignment of
ZIP codes (USA postal codes) to geographical location is an
arbitrary encoding -- some large-scale patterns are evident
but they're not regular enough that you could derive map
coordinates from numerical calculations on the codes.[*]
May as well treat them as arbitrary keys and look them up
in a table.

    Extrapolation doesn't seem to enter the picture, though.

    [*] Mathematically, of course, one could always build a
polynomial of degree 100000 or less that interpolates every
five-digit ZIP code to its latitude, and another polynomial
for longitude.  You might, however, have some difficulty in
evaluating such high-degree polynomials with useful accuracy!
In fact, I rather suspect that simply storing the coefficients
(as pairs of BigIntegers representing rational numbers, say)
might take more memory than the average 32-bit JVM can supply.

Thanks, all, for the advice.  I recently learned that there are some
places that sell a file with all US Zip codes and their GPS coords.  I
wonder if I could make use of that?  Hmmm...

Anyway, thanks, again, for your thoughts!

Korto

Looks like the post office uses the Google Maps API to convert it for
them
http://www.zip-codes.com/