Index on a function and SELECT DISTINCT

If I have this table, function and index in Postgres 7.3.6 ...

"""
CREATE TABLE news_stories (
    id serial primary key NOT NULL,
    pub_date timestamp with time zone NOT NULL,
    ...
)
CREATE OR REPLACE FUNCTION get_year_trunc(timestamp with time zone) returns
timestamp with time zone AS 'SELECT date_trunc(\'year\',$1);' LANGUAGE 'SQL'
IMMUTABLE;
CREATE INDEX news_stories_pub_date_year_trunc ON
news_stories( get_year_trunc(pub_date) );
"""

...why does this query not use the index?

db=# EXPLAIN SELECT DISTINCT get_year_trunc(pub_date) FROM news_stories;
                                   QUERY PLAN
---------------------------------------------------------------------------------
 Unique  (cost=59597.31..61311.13 rows=3768 width=8)
   ->  Sort  (cost=59597.31..60454.22 rows=342764 width=8)
         Sort Key: date_trunc('year'::text, pub_date)
         ->  Seq Scan on news_stories  (cost=0.00..23390.55 rows=342764
width=8)
(4 rows)

The query is noticably slow (2 seconds) on a database with 150,000+ records.
How can I speed it up?

Thanks,
Adrian