Immutable attributes?

To protect the database from programming errors (there is a team
working on the project and some beginners may produce bugs), I would
like to flag some attributes as immutable, meaning non modifiable in
an UPDATE. (Typical examples are ID or creation time.)

Currently, I use triggers:

CREATE OR REPLACE FUNCTION check_immutable() RETURNS TRIGGER
    AS 'BEGIN
              IF NEW.id != OLD.id OR NEW.created != OLD.created THEN
                   RAISE EXCEPTION ''Change not allowed in that table'';
              END IF;
          RETURN NEW;
        END;'
    LANGUAGE PLPGSQL;

CREATE TRIGGER check_immutable
   BEFORE UPDATE ON MyTable
   FOR EACH ROW
   EXECUTE PROCEDURE check_immutable();

It is quite painful, since I need a function (with the list of
immutable attributes) and a trigger per table. If I INHERITS from a
table, triggers on the parent table are not called if I update the
child table.

Is there a better way? I do not find a constraint IMMUTABLE for
attributes.

PostgreSQL 7.4, switching to 8.0 would be difficult.