Index: wp-includes/meta.php
===================================================================
--- wp-includes/meta.php	(revision 24974)
+++ wp-includes/meta.php	(working copy)
@@ -151,16 +151,20 @@
 	if ( 'post' == $meta_type )
 		do_action( 'update_postmeta', $meta_id, $object_id, $meta_key, $meta_value );
 
-	$wpdb->update( $table, $data, $where );
+	$updated = $wpdb->update( $table, $data, $where );
 
-	wp_cache_delete($object_id, $meta_type . '_meta');
+	if ( $updated ) {
+		wp_cache_delete($object_id, $meta_type . '_meta');
 
-	do_action( "updated_{$meta_type}_meta", $meta_id, $object_id, $meta_key, $_meta_value );
+		do_action( "updated_{$meta_type}_meta", $meta_id, $object_id, $meta_key, $_meta_value );
 
-	if ( 'post' == $meta_type )
-		do_action( 'updated_postmeta', $meta_id, $object_id, $meta_key, $meta_value );
+		if ( 'post' == $meta_type )
+			do_action( 'updated_postmeta', $meta_id, $object_id, $meta_key, $meta_value );
 
-	return true;
+		return true;
+	} else {
+		return false;
+	}
 }
 
 /**