$(document).ready(function () {  
  
  var $comment = $('#comment'),
      top = $comment.offset().top - parseFloat($comment.css('marginTop').replace(/auto/, 0)),
      $window = $(window);
              
  $window.scroll(function (event) {
    // what the y position of the scroll is
    var y = $window.scrollTop();
  
    // whether that's below the form
    if (y >= top) {
      // if so, ad the fixed class
      $comment.addClass('fixed');
    } else {
      // otherwise remove it
      $comment.removeClass('fixed');
    }
  });
});