Index: wp-includes/canonical.php
===================================================================
--- wp-includes/canonical.php	(revision 412)
+++ wp-includes/canonical.php	(working copy)
@@ -92,8 +92,7 @@
 			}
 		}
 
-		if ( ! $redirect_url )
-			$redirect_url = redirect_guess_404_permalink();
+		$redirect_url = apply_filters( '404_guess_permalink', $redirect_url );
 
 	} elseif ( is_object($wp_rewrite) && $wp_rewrite->using_permalinks() ) {
 		// rewriting of old ?p=X, ?m=2004, ?m=200401, ?m=20040101
@@ -409,6 +408,8 @@
 	return get_permalink($post_id);
 }
 
+add_filter( '404_guess_permalink', 'redirect_guess_404_permalink' );
+
 add_action('template_redirect', 'redirect_canonical');
 
 ?>