sample_jsonp_controller.rb

before_filter :validate_request_format

# ...

def send_response data  
  respond_with(data) do |format|
    format.js   { render json: data, callback: params[:callback] } # JSONP
    format.json { render }
    format.xml  { render }
  end
end

def validate_request_format
  if request.format == Mime::JS && params[:callback].blank?
    refuse_request
  end
end

def refuse_request
  head :bad_request
end