Concurrency and locks

Is it possible, or could it be made possible in the future, for
a ROW locking mechanism whereby:

Transaction #1

BEGIN;
SELECT key FROM foo WHERE key = 1 FOR ????;

returns immediately with '1'

Transaction #2

BEGIN;
SELECT key FROM foo WHERE key = 1 FOR ????;

returns immediately with '1' <-- Not blocked by #1

Transaction #3

BEGIN;
UPDATE foo SET key = 2 WHERE key = 1; <-- Blocks

If one wants to guarantee consistency in user-defined BEFORE
INSERT/UPDATE triggers and trigger procedures, at the moment, is
the only recourse SELECT FOR UPDATE? Is this the cause of
performance problems with the current RI implementation?

Mike Mascari
mascarm(at)mascari(dot)com