diff --git includes/form.inc includes/form.inc
--- includes/form.inc	Tue Jan 04 16:46:24 2011
+++ includes/form.inc	Thu Apr 21 00:42:41 2011
@@ -2521,9 +2521,13 @@
  */
 function theme_select($variables) {
   $element = $variables['element'];
-  element_set_attributes($element, array('id', 'name', 'size'));
+  $attributes = array('id','name','size');
+  if (isset($element['#multiple']) && $element['#multiple']==true) {
+    $element['#attributes'] += array('multiple'=>'multiple');
+    $element['#name'] = $element['#name'].'[]';
+  }
+  element_set_attributes($element, $attributes);
   _form_set_class($element, array('form-select'));
-
   return '<select' . drupal_attributes($element['#attributes']) . '>' . form_select_options($element) . '</select>';
 }