$(document).ready(function() {
 var options_feedback = {
  target: '#feedback',
  beforeSubmit: loading,
  success: loaded  
 };
 $('#feedback_form').ajaxForm(options_feedback);
});

function loading() {
 $('#submit').attr('disabled','disabled');
 return true;
};

function loaded() {
 $('#submit').removeAttr('disabled');
 return true;
};