Function Volatility

Hi guys,

I am not sure if I am understanding volatility.
My issue is better explained with a quick example. The function below
expresses call durations in minutes and it is immutable.

CREATE OR REPLACE FUNCTION dur2min(secs INTEGER) RETURNS INTEGER
AS $$
BEGIN
RAISE NOTICE 'BEEN HERE!';
RETURN CEIL(secs/60.0);
END;
$$ LANGUAGE 'plpgsql' IMMUTABLE;

# SELECT dur2min(30) as c1, dur2min(30) as c2, dur2min(30) as c3;

NOTICE: BEEN HERE!
NOTICE: BEEN HERE!
NOTICE: BEEN HERE!
c1 | c2 | c3
----+----+----
1 | 1 | 1
(1 row)

What bother me are the 3 "been here" messages. As the function is immutable
and the parameter remains unchanged needs the planner actually execute the
function 3 times?
I was under the impression that under these conditions it could *reuse* the
result of the first call. The manual states the planner should avoid
reevaluate the function but I'm not sure what that means as it *is*
executing it every time.

My goal of course is that the function gets executed only once per row.
I'm using 8.2.4

Thanks for your hindsight.
Regards,
Fernando.