Sequential scan where Index scan expected (update)

I'm getting sequential scans (and poor performance), on scans using my
primary keys. This is an older postgres.
Can anyone help figure out why?

demo=# \d xx_thing
Table "public.xx_thing"
Column | Type | Modifiers
-------------------------+-----------------------------+-----------
thing_id | bigint | not null
thing_model | character varying(128) |
thing_color | character varying(128) |
thing_year | integer |
Indexes:
"xx_thing_pkey" primary key, btree (thing_id)

demo=# analyze verbose xx_thing_event;
INFO: analyzing "public.xx_thing_event"
INFO: "xx_thing_event": 3374 pages, 3000 rows sampled, 197478 estimated
total rows

demo=# explain update xx_thing_event set thing_color='foo' where
thing_event_id=10000;
QUERY PLAN
---------------------------------------------------------------------
Seq Scan on xx_thing_event (cost=0.00..5842.48 rows=1 width=110)
Filter: (thing_event_id = 10000)
(2 rows)

demo=# select * from version();
version
----------------------------------------------------------------------------------------------------------
PostgreSQL 7.4.1 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.2.3
20030502 (Red Hat Linux 3.2.3-20)
(1 row)