--- form.inc	2011-03-20 19:06:20.000000000 +0100
+++ form_new.inc	2011-03-20 19:08:32.000000000 +0100
@@ -2633,6 +2633,18 @@ function form_get_options($element, $key
  */
 function theme_fieldset($variables) {
   $element = $variables['element'];
+  if (!empty($element['#collapsible'])) {
+    drupal_add_library('system', 'drupal.collapse');
+
+    if (!isset($element['#attributes']['class'])) {
+      $element['#attributes']['class'] = array();
+    }
+
+    $element['#attributes']['class'][] = 'collapsible';
+    if (!empty($element['#collapsed'])) {
+      $element['#attributes']['class'][] = 'collapsed';
+    }
+  }
   element_set_attributes($element, array('id'));
   _form_set_class($element, array('form-wrapper'));