www.rb

class WWW

  STARTS_WITH_WWW = /^www\./i

  def initialize(app)
    @app = app
  end

  def call(env)
    if env['HTTP_HOST'] =~ STARTS_WITH_WWW
      @app.call(env)
    else
      [301, { 'Location' => Rack::Request.new(env).url.sub(/^http:\/\//i, 'http://www.') }, ['Redirecting...']]
    end
  end

end

environments/production.rb:
config.middleware.use WWW