flagging first row inserted for each "group" of key

I've a web application.

I'm logging data related to the landing page and subsequent selected
hits.

create table track_hit (
  hitid serial not null,
  /* pk? I don't mind if serial wrap around
  pk could be (hitid, tracking_time) */
  esid varchar(32), -- related to session
  tracking_time timestamp not null default now(),
  -- some other stats
  -- first_hit boolean not null default false, -- ???
);

I'd like to be sure I just count one "first hit" in a session (same
esid) in an efficient way that means I'd like to mark them with a
flag and avoid a group by, min subquery cycle when I'm doing
reporting.
I can't trust the browser and I just want one first hit for each
esid, no matter if they have the same tracking_time.

Of course the problem is concurrency, but maybe I missed some pg
trick that could help me.

I'm on 8.3 and no plan to move to 8.4 shortly, so no windowing
functions that will make reporting easier/faster.

--
Ivan Sergio Borgonovo
http://www.webthatworks.it