some_controller.rb

class SomeController < ApplicationController
  # http://blog.jetthoughts.com/2010/12/22/allow-multiple-access-control-requests-for-rails/
  before_filter :set_access_control_headers
  
  def show
    @thing = Thing.find(params[:id])
    respond_to do |format|
      format.html
      format.json do
        body = render_to_string(:locals => {:thing => @thing})
        render :json => "{ \"html\" :  \"#{to_json_value(body)}\"}", :callback => params[:callback]
      end
    end
  end
  
  # this could either go in ApplicationController or
  # create an instance method on String
  def to_json_value(str)
    str.gsub!("\"", "\\\"")
    str.gsub!(/\n+/, " ")
    str
  end

end