Index: wp-includes/meta.php
===================================================================
--- wp-includes/meta.php	(revision 12676)
+++ wp-includes/meta.php	(working copy)
@@ -13,6 +13,9 @@
 	if ( !$meta_type || !$meta_key )
 		return false;
 
+	if ( !$object_id = absint($object_id) )
+		return false;
+
 	if ( ! $table = _get_meta_table($meta_type) )
 		return false;
 
@@ -47,6 +50,9 @@
 	if ( !$meta_type || !$meta_key )
 		return false;
 
+	if ( !$object_id = absint($object_id) )
+		return false;
+
 	if ( ! $table = _get_meta_table($meta_type) )
 		return false;
 
@@ -82,9 +88,12 @@
 }
 
 function delete_metadata($meta_type, $object_id, $meta_key, $meta_value = '', $delete_all = false) {
-	if ( !$meta_type || !$meta_key || (!$delete_all && ! (int)$object_id) )
+	if ( !$meta_type || !$meta_key )
 		return false;
 
+	if ( !$delete_all && !$object_id = absint($object_id) )
+		return false;
+
 	if ( ! $table = _get_meta_table($meta_type) )
 		return false;
 
@@ -126,6 +135,9 @@
 	if ( !$meta_type )
 		return false;
 
+	if ( !$object_id = absint($object_id) )
+		return false;
+
 	$meta_cache = wp_cache_get($object_id, $meta_type . '_meta');
 
 	if ( !$meta_cache ) {