First BDD/RSpec tests, would like some review regarding idioms, conventions and style
A small hint: the count method could clean up one of your test case, more or less as such:

it "should have more than half the tiles be landmass" do  
    map = @map_factory.make(40, 40)
    number_of_water_tiles = map.tiles.count{|tile| tile.type == :water}
    number_of_land_tiles = map.tiles.size - number_of_water_tiles
    number_of_land_tiles.should be &gt; (map.tiles.size / 2)
  end
end


Also your map could go in the before section too imho.