Index: common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.1082
diff -u -p -r1.1082 common.inc
--- common.inc	8 Jan 2010 06:07:03 -0000	1.1082
+++ common.inc	9 Jan 2010 03:49:26 -0000
@@ -3205,6 +3205,7 @@ function drupal_add_css($data = NULL, $o
       'media' => 'all',
       'preprocess' => TRUE,
       'data' => $data,
+      'modified' => filemtime($data),
     );
 
     // Always add a tiny value to the weight, to conserve the insertion order.