For a given latitude, longitude and zoom level return the Google maps tile reference for that location - as documented at http://code.google.com/apis/maps/documentation/javascript/v2/overlays.html#Projections

tile_info.rb

def tile_info(lat, lon, zoom)
  rad_lat = lat / 180 * Math::PI
  rad_lon = lon / 180 * Math::PI

  merc_x = rad_lon
  merc_y = Math.log( Math.tan(rad_lat) + 1/ Math.cos(rad_lat) )

  cart_x = merc_x + Math::PI
  cart_y = Math::PI - merc_y

  px0 = cart_x * 256 / (2 * Math::PI )
  py0 = cart_y * 256 / (2 * Math::PI )
  tile_x = px0 * (2 ** zoom) / 256
  tile_y = py0 * (2 ** zoom) / 256
  {:x => tile_x.floor, :y => tile_y.floor}
end