function initMaxlengthTextAreas() {
  $$('textarea').each(function(ta) {
    if (ta.readAttribute('maxlength')) {
      ta.observe('keyup', function(event) {
        el = event.element();
        val = el.value;
        max = parseInt(el.readAttribute('maxlength'));
        if (val.length > max) {
          el.value = val.substring(0,max);
        }
      });
    }
  });
}

Event.observe(window, 'load', initMaxlengthTextAreas);