0.html

<script type="text/javascript">
function parseUser (json) {
  var user = new User(json)
  // do something with the user

}
</script>

<script type="text/javascript"
        src="http://server2.example.com/users/1.js?callback=parseUser">
</script>

1.rb

gem 'rack-cors',
  :require => 'rack/cors'

2.rb

module Sample
  class Application < Rails::Application
    # other application config

    config.middleware.use Rack::Cors do
      allow do
        origins 'server1.example.com'
        resource %r{/users/\d+.json},
          :headers => ['Origin', 'Accept', 'Content-Type'],
          :methods => [:put, :delete]
      end
    end
  end
end

3.rb

class UsersController < ApplicationController
  def show
    respond_to do |format|
      format.js do
        @user = User.find params[:id]
        render :json => @user, :callback => params[:callback]
      end
    end
  end
end

4.sh

$ curl http://localhost:3000/users/1.js\?callback\=parseUser
parseUser({"age":50,"created_at":"2012-02-22T05:44:28Z","id":1,"name":"user #1","updated_at":"2012-02-22T05:44:28Z"})

5.rb

gem 'rack-jsonp-middleware',
  :require => 'rack/jsonp'

6.rb

module Sample
  class Application < Rails::Application

    # other application config

    config.middleware.use Rack::JSONP
  end
end

7.html

<script type="text/javascript"
        src="http://server2.example.com/users/1.jsonp?callback=parseUser">
</script>

8.rb

class UsersController < ApplicationController
  respond_to :json

  def show
    @user = User.find params[:id]
    respond_with @user
  end
end

9.sh

$ curl http://localhost:3000/users/1.jsonp\?callback\=parseUser
parseUser({"age":50,"created_at":"2012-02-22T05:44:28Z","id":1,"name":"user #1","updated_at":"2012-02-22T05:44:28Z"});