Index: wp-includes/author-template.php
===================================================================
--- wp-includes/author-template.php	(revision 18641)
+++ wp-includes/author-template.php	(working copy)
@@ -385,7 +385,7 @@
 		wp_cache_set('is_multi_author', $is_multi_author, 'posts');
 	}
 
-	return (bool) $is_multi_author;
+	return apply_filters( 'is_multi_author', (bool) $is_multi_author );
 }
 
 /**