I don't personally have any samples (anymore) but several years ago I was considering doing something like this for another application.  I can tell you the basic theory behind it and give some terms that you could Google:

Essentially, doing calculations between zipcodes is a rough estimate to their epicenter based on latitude and longitude.  If you wanted to perform the calculation directly in your DNN site you'd need a way to store the latitude and longitude for all of the zip codes and then use some nifty code to triangulate the distance.  An example of the calculations can be found on CodeProject while tutorials (and possibly some sample web services) can be found on ZipCodeWorld. 

Additionally, check out the following webservice at http://webservices.imacination.com/distance/ ; it seems to be free and provides that accomodation.  Another example http://teachatechie.com/GJTTWebservices/ZipCode.asmx

The problem with calling web services from within DNN is that it requires some additional permissions that may not work in a medium trust server.  If you are running your own server this isn't so much a problem but if you are running from a hosted server this can (and likely will) become an issue.

Hope that helps.