I asked the same question several weeks ago. Google's reverse geocoding api is a great solution. Instead of inserting hundreds of thousands of records into a database and juggling different tedious validation techniques it's a much better idea to only use latitudes and longitudes for locating people geographically. However, the project's details may require other routes but still thought I would share:

http://code.google.com/apis/maps/documentation/examples/geoc...

Users just click on their location on the map and Google hands off the nearest address info.