stored procedure and timestamp

I have the following trigger/stored procedure:

CREATE FUNCTION memb_num () RETURNS opaque AS '
         BEGIN
                 NEW.member_num := date_part(''epoch'', timestamp ''now'');
                 RETURN NEW;
         END;
' LANGUAGE 'plpgsql';

CREATE TRIGGER memb_num_trigger BEFORE INSERT
         ON members FOR EACH ROW
         EXECUTE PROCEDURE memb_num();


I get the following error when I attempt to insert any records beyond the
first:
"Cannot insert a duplicate key into unique index members_member_num_key"

So the function isn't inserting "live" timestamps. I believe this means the
function is not trying to set NEW.member_num to the timestamp of when the
function gets called, which is what I expected. I'm guessing the function
is evaluating the timestamp at the moment of adding the function to the
database - then inserting that stored timestamp repeatedly?