On my website I would like to include a radius search for user profiles, similar to what Twitter is doing on their advanced search with "Near this places"
I currently have a US zip code database with lat & lon data, but the site is now expanding globally. What's the best approach to adding local proximity search all around the world? Adding a zip code table for every country? How does Twitter do it?
What you need is a Geocoding service. There are a number of options both free and paid for. Google offer a Geocoder as part of their maps API, though rate restrictions apply, and the data must be plotted on a Google map.
e.g. http://maps.google.com/maps/api/geocode/json?address=[insert+address/postcode+here]&sensor=false
Another free option is opengeocoding.org. there are also a number of commercial options.