Index: wp-includes/post-thumbnail-template.php
===================================================================
--- wp-includes/post-thumbnail-template.php	(revision 32330)
+++ wp-includes/post-thumbnail-template.php	(working copy)
@@ -31,7 +31,7 @@
  */
 function get_post_thumbnail_id( $post_id = null ) {
 	$post_id = ( null === $post_id ) ? get_the_ID() : $post_id;
-	return get_post_meta( $post_id, '_thumbnail_id', true );
+	return apply_filters( 'post_thumbnail_id', get_post_meta( $post_id, '_thumbnail_id', true ), $post_id );
 }

 /**