According to the Google geo dev blog this week, Google now is making reverse geocoding available to developers. This is pretty huge for developers... in a nutshell this enables developers to add functionality that generates an address from a lat/long pair - a much tougher process than the reverse, which is grabbing a coordinate pair from an address (geocoding).
As a great example of a cool use of this technology enter MeetWays, a clever mashup that enables users to enter 2 locations (detailed or simply using zip codes) along with a variable (like pizza or coffee etc...) and in return you get returned locations that are aprrox in the middle... imagine, I enter your zip and mine and we determine a half way point to meet-up.. this is awesome!!
See also the HTTP Geocoding service for server-side geocoding, and the GClientGeocoder class for client-side geocoding
Google Geo Developers Blog: Geocoding... in Reverse!