On the Etsy "Code as Craft" blog there's a recent post about their move to using the GeoNames service internally rather than the external, third-party API previously in use.

People are passionate about where they live. At Etsy we need to keep track of lots of different locations, such as buyers’ billing and shipping addresses and sellers’ shop locations. As Etsy continues to expand internationally we wanted to provide better localization and translations for our location place names. We determined that the best way to effect this change was to move from using a closed location API provider to internal services backed by the open GeoNames data set.

The post details some of the steps in the process including the mapping of the current data to the new structure (the script is available on github). They talk about how they mapped their old data over (trial and error sometimes) and the creation of a database of "GeoNameIDs" for each customer in their records. They've also implemented a Lucene/Solr search for improved searching and auto-suggestion based on the user's location.