simplified version of reverse geocode parse

post.rb

require "json"
require "open-uri"

class Post
  def self.location_label(latitude, longitude)
      data = open("http://nominatim.openstreetmap.org/reverse?format=json&lat=#{latitude}&lon=#{longitude}&zoom=18&addressdetails=1").readlines.join("\n")
      result = JSON.parse(data)
      result_arr = []

      # london mode
      if result["address"].has_key?("county") && result["address"]["county"] == "Greater London"
        if result["address"].has_key?("place")
          result_arr << result["address"]["place"]
        else
          if result["address"].has_key?("pedestrian")
            result_arr << result["address"]["pedestrian"] 
          elsif result["address"].has_key?("footway")
            result_arr << result["address"]["footway"]
          end
          result_arr << result["address"]["suburb"] if result["address"].has_key?("suburb")
        end
        # state_district should be 'London' :)
        result_arr << result["address"]["state_district"] if result["address"].has_key?("state_district")

        # strip end off postcodes
        if result["address"].has_key?("postcode")
          postcode = result["address"]["postcode"]
          if postcode.split(" ").size != 1
            result_arr << postcode.split(" ").first
          else
            result_arr << postcode
          end
        end
      # not london.
      else
        if result["address"].has_key?("road")
          result_arr << result["address"]["road"]
        end
        if result["address"].has_key?("suburb")
          result_arr << result["address"]["suburb"]
        end

        if result["address"].has_key?("town")
          result_arr << result["address"]["town"]
        else
          if result["address"].has_key?("city")
            result_arr << result["address"]["city"]
          end
        end
      end

      return result_arr.join(", ")
  end
end

post_test.rb

class PostTest
  test "posts generate useful location labels" do
    assert_equal "St Pancras, London, NW1", Post.location_label(51.5294418333333,-0.126272633333333)
    assert_equal "St Pancras, London, NW1", Post.location_label(51.5295385,-0.1264996)
    assert_equal "Bishops Square, London, E1", Post.location_label(51.5192044,-0.077819)
    assert_equal "Westminster, London, WC2N", Post.location_label(51.509092275,-0.123459925)
    assert_equal "Gloucester Road, Round Hill, Brighton", Post.location_label(50.8267737,-0.1367264)
    assert_equal "Trinity Street, Bohemia, Hastings", Post.location_label(50.8550399,0.5767676)
  end
end

As @danstowell has produced a Python version, I should probably license this code. It's under the MIT License.

Copyright © 2012 Tom Morris.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.