R – Get street address at lat/long pair

geocodinggisiphonestreet-addressweb-services

I've seen that it's possible to get the latitude and longitude (geocoding, like in Google Maps API) from a street address, but is it possible to do the reverse and get the street address when you know what the lat/long already is?

The application would be an iPhone app (and why the app already knows lat/long), so anything from a web service to an iPhone API would work.