LARGE table won't use index?

I'm having a problem getting PostgreSQL 7.2.1 to use an index when it
is painfully obvious that the index will be faster. Here is the
situation:

2 tables:

1) keyword
keyword_id integer primary key
word varchar(50)

- the word is indexed (keyword_word_index)
- the table contains ~100k rows

2) article_keyword (this is a mapping table for a many-many)
article_id integer
keyword_id integer
relevenace integer
primary_key(article_id, keyword_id)

- the keyword_id col is indexed (ak_kwid_index)
- the table contains ~2 million rows.

OK. Consider the follwing psql/explain output:
-------------------------------------------------------------------------
search.dev=# select * from keyword where word='spam';
keyword_id | word
------------+------
2616 | spam
(1 row)

search.dev=# explain select * from article_keyword where
keyword_id=2616;
NOTICE: QUERY PLAN:

Index Scan using ak_kwid_index on article_keyword (cost=0.00..2990.85
rows=810 width=12)

EXPLAIN
search.dev=# explain select * from article_keyword where keyword_id in
search.dev-# (select keyword_id from keyword where word='spam');
NOTICE: QUERY PLAN:

Seq Scan on article_keyword (cost=0.00..3378740757.90 rows=1820389
width=12)
SubPlan
-> Materialize (cost=1856.04..1856.04 rows=1345 width=4)
-> Index Scan using keyword_word_index on keyword
(cost=0.00..1856.04 rows=1345 width=4)

EXPLAIN
---------------------------------------------------------------------------

Note that when I query using the explicit keyword_id, the query plan
uses the index at a cost of about 3000. When I use a subselect (which
returns the EXACT same keyword_id and I explicitly used) the plan uses
a sequence scan of 2 million rows at a cost of about 3.3 billion!!

I really need for postgres to use the indexes on the mapping table,
what can I do. Any help from the PostgreSQL gods is appreciated.

Paul Bemowski
bemowski(at)yahoo(dot)com