Index: wp-includes/meta.php
===================================================================
--- wp-includes/meta.php	(revision 18324)
+++ wp-includes/meta.php	(working copy)
@@ -588,8 +588,12 @@
  * @return bool True if the key is protected, false otherwise.
  */
 function is_protected_meta( $meta_key, $meta_type = null ) {
-	$protected = (  '_' == $meta_key[0] );
+	$protected = false;
+	$protected_keys = array( '_edit_last', '_edit_lock', '_thumbnail_id' );
 
+ 	if ( 0 === strpos( $meta_key, '_wp_' ) || 0 === strpos( $meta_key, '_menu_item_' ) || in_array( $meta_key, $protected_keys ) )
+		$protected = true;
+
 	return apply_filters( 'is_protected_meta', $protected, $meta_key, $meta_type );
 }