Why isn't this index being used?

The following is from a database of several hundred million rows of real
data that has been VACUUM ANALYZEd.

Why isn't the index being used for a query that seems tailor-made for
it? The results (6,300 rows) take about ten minutes to retrieve with a
sequential scan.

A copy of this database with "integer" in place of "smallint", a primary
key in column order (date, time, type, subtype) and a secondary index in
the required order (type, subtype, date, time) correctly uses the
secondary index to return results in under a second.

Actually, the integer version is the first one I made, and the smallint
is the copy, but that shouldn't matter.

Postgres is version "postgresql-server-7.3.4-3.rhl9" from Red Hat Linux
9.

=====

testdb2=# \d db

Table "public.db"

Column | Type | Modifiers

---------+------------------------+-----------

date | date | not null

time | time without time zone | not null

type | smallint | not null

subtype | smallint | not null

value | integer |

Indexes: db_pkey primary key btree ("type", subtype, date, "time")

testdb2=# set enable_seqscan to off;

SET

testdb2=# explain select * from db where type=90 and subtype=70 and
date='7/1/2004';

QUERY PLAN

------------------------------------------------------------------------
------

Seq Scan on db (cost=100000000.00..107455603.76 rows=178 width=20)

Filter: (("type" = 90) AND (subtype = 70) AND (date =
'2004-07-01'::date))

(2 rows)