Automatically updating a timestamp upon UPDATE query?

How do you most easily create a timestamp column that updates to the
current time on every UPDATE statement?  I know you can use triggers, but
I thought there was a default value that did this for you.  'current'
does not seem appropriate, nor have I made it work (see below for my
example).

    http://www.postgresql.org/docs/postgres/x1137.htm :
    "'now' is resolved when the value is inserted, 'current' is
     resolved everytime the value is retrieved."

Any clues?

Regards,
Ed Loehr

BTW, here's the sequence I used to conclude 'current' doesn't seem to
work:

DROP TABLE foo;
CREATE TABLE foo (
    note    VARCHAR,
    created TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    updated TIMESTAMP NOT NULL DEFAULT 'current'
);

SELECT now();
INSERT INTO foo (note) VALUES ('Default timestamp test record');
SELECT created FROM foo;
SELECT now();
SELECT * FROM foo;

--  Wait a few seconds for the time to clearly change seconds...

SELECT now();
UPDATE foo SET note = 'Did the update_time change with an UPDATE query?';
SELECT now();
SELECT * FROM foo;
SELECT updated FROM foo;