Index: includes/common.inc
===================================================================
--- includes/.svn/text-base/common.inc.svn-base	2007-05-24 16:02:23.000000000 -0400
+++ includes/common.inc	2007-05-25 12:36:13.000000000 -0400
@@ -1427,11 +1427,19 @@ function drupal_add_css($path = NULL, $t
   if (isset($path)) {
     // This check is necessary to ensure proper cascading of styles and is faster than an asort().
     if (!isset($css[$media])) {
       $css[$media] = array('module' => array(), 'theme' => array());
     }
-    $css[$media][$type][$path] = $preprocess;
+    if ($preprocess && variable_get('preprocess_css', FALSE)) {
+      if (!isset($css[$media][$type][$path])) {
+        // using is_file() first avoids a warning in filemtime() if the file doesn't exist
+        $css[$media][$type][$path] = is_file($path) ? filemtime($path) : FALSE;
+      }
+    }
+    else {
+      $css[$media][$type][$path] = FALSE;
+    }
   }
 
   return $css;
 }