include_root_in_json.rb

User.include_root_in_json = true
User.first.to_json(:only => [:screen_name,:id])
#=> {"user":{"id":1,"screen_name":"test"}}
User.include_root_in_json = false
User.first.to_json(:only => [:screen_name,:id])
#=> {"id":1,"screen_name":"test"}

inline_json.rb

class UsersController < ApplicationController::Base
  def index
    @users = User.all
    respond_to do |format|
      format.html
      format.json { render :json => @users }
    end
  end
end

inline_jsonp.rb

render :json => @users.to_json, :callback => params[:callback]
#=> jsonp1288721752692(["user1", "user2"]);

respond_with_json.rb

class UsersController < ApplicationController::Base
  respond_to :html, :json

  def index
    respond_with(@users = User.all)
  end
end