--- includes/form.inc	2011-01-04 18:46:24.000000000 +0200
+++ includes/form.inc	2011-03-21 15:07:02.000000000 +0200
@@ -3031,7 +3031,7 @@
         '#type' => 'checkbox',
         '#title' => $choice,
         '#return_value' => $key,
-        '#default_value' => isset($value[$key]) ? $key : NULL,
+        '#default_value' => isset($value[$key]) || (is_array($element['#default_value']) && in_array($key, $element['#default_value'])) ? $key : NULL,
         '#attributes' => $element['#attributes'],
         '#ajax' => isset($element['#ajax']) ? $element['#ajax'] : NULL,
         '#weight' => $weight,