Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 13906)
+++ wp-includes/theme.php	(working copy)
@@ -1357,7 +1357,10 @@
 function register_default_headers( $headers ) {
 	global $_wp_default_headers;
 
-	$_wp_default_headers = $headers;
+	if ( is_array($_wp_default_headers) )
+		$_wp_default_headers = array_merge($_wp_default_headers, $headers);
+	else
+		$_wp_default_headers = $headers;
 }
 
 /**