Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 7696)
+++ wp-includes/pluggable.php	(working copy)
@@ -1040,9 +1040,10 @@
  */
 function wp_salt() {
 	global $wp_default_secret_key;
-	$secret_key = '';
 	if ( defined('SECRET_KEY') && ('' != SECRET_KEY) && ( $wp_default_secret_key != SECRET_KEY) )
 		$secret_key = SECRET_KEY;
+	else
+		$secret_key = md5(filemtime(ABSPATH . 'wp-config.php'));
 
 	if ( defined('SECRET_SALT') ) {
 		$salt = SECRET_SALT;