Index: includes/form.inc
===================================================================
--- includes/form.inc	(drupal 5.1)
+++ includes/form.inc	(working copy)
@@ -1531,7 +1531,7 @@
  *   A string representing the form element.
  */
 function theme_form_element($element, $value) {
-  $output  = '<div class="form-item">'."\n";
+  $output  = '<div class="form-item'.($element['#required']?' required':'').(form_get_error($element)?' error':'').'">'."\n";
   $required = !empty($element['#required']) ? '<span class="form-required" title="'. t('This field is required.') .'">*</span>' : '';
 
   if (!empty($element['#title'])) {