Behavior of nextval() and currval()

The following works in 7.1. Is this behavior guaranteed or is it the
subject of possible change in the future? Or am I just "getting
lucky" that nextval() is evaluated before currval():

CREATE SEQUENCE s;

CREATE TABLE test (
key1 int4,
key2 int4
);

INSERT INTO test
SELECT nextval('s'), currval('s');

SELECT * FROM test;

key1 | key2
------+------
1 | 1

Thanks for any information,

Mike Mascari
mascarm(at)mascari(dot)com