=== modified file 'includes/form.inc'
--- includes/form.inc	2006-10-18 11:43:27 +0000
+++ includes/form.inc	2006-10-19 23:25:45 +0000
@@ -1195,7 +1195,11 @@ function theme_checkbox($element) {
  *   A themed HTML string representing the checkbox set.
  */
 function theme_checkboxes($element) {
-  $element['#children'] = '<div class="form-checkboxes">'. $element['#children'] .'</div>';
+  $class = 'form-checkboxes';
+  if (isset($element['#attributes']['class'])) {
+    $class .= ' '. $element['#attributes']['#class'];
+  }
+  $element['#children'] = '<div class="'. $class .'">'. $element['#children'] .'</div>';
   if ($element['#title'] || $element['#description']) {
     unset($element['#id']);
     return theme('form_element', $element, $element['#children']);