Selective Auto-Timestamp [using triggers?]

Rajit Singh writes:
> The thing is, I also want to be able to update the timestamp field
> if I want to...

Here's my solution:

CREATE FUNCTION updated_stamp () RETURNS OPAQUE AS
'    BEGIN
    IF NEW.updated ISNULL THEN
        NEW.updated := ''now'';
    END IF;
        RETURN NEW;
    END;
' LANGUAGE 'plpgsql';