Making NULL entries appear first when ORDER BY <field> ASC

Hi, I have the following table, with the query below to list entries from it
where start_date IS NOT NULL:

CREATE TABLE onp_crm_activity_log(
id serial PRIMARY KEY,
start_date timestamp,
start_time timestamp,
end_time timestamp,
title varchar NOT NULL
);

SELECT start_date, start_time, end_time, title
FROM onp_crm_activity_log
WHERE start_date IS NOT NULL
ORDER BY start_date ASC, start_time ASC;

start_date | start_time | end_time | title
---------------------+---------------------+---------------------+-----------
2005-02-03 00:00:00 | 2005-02-03 08:00:00 | | Something
2005-02-03 00:00:00 | 2005-02-03 09:00:00 | 2005-02-03 12:00:00 | Something
2005-02-03 00:00:00 | | | Something

Now, as you see, touples with NULL in the "start_time"-field appear "after"
the others. I would like to make all entries where start_time IS NULL apear
*before* all the others. Any idea how to achieve this?

--
Andreas Joseph Krogh <andreak(at)officenet(dot)no>
Senior Software Developer / Manager
gpg public_key: http://dev.officenet.no/~andreak/public_key.asc
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