Need Some Explanation of an EXPLAIN

I'm trying to update a field in a table that has about 5 millin records
with a table that has about 3.5 million records. I've created indexes
for th a joined columns, but PostgreSQL 8.1 doesn't seem to want to use
them. This makes for a very slow update.

Below are descriptions of the two tables followed by the query plan that
my PostgreSQL wants to use. So I have two questions:

1.) Why won't it use the indexes?
2.) How can I make this update faster?

TIA,
Bill Thoen

Table "public.id2"
Column | Type | Modifiers
-----------+--------------+-----------
grower_id | integer |
fmid | character(7) |
fsa_id | character(9) |
Indexes:
"id2_fsa_is_key" UNIQUE, btree (fsa_id)

Table "public.growers"
Column | Type | Modifiers
--------------+-----------------------+-----------
grower_id | integer |
fsa_id | character(9) |
co_name | character varying(45) |
. . .
Indexes:
"grower_fsa_id_key" btree (fsa_id)

fsa=# EXPLAIN UPDATE growers
SET grower_id = id2.grower_id
FROM id2 WHERE growers.fsa_id = id2.fsa_id;

QUERY PLAN
--------------------------------------------------------------------------
Hash Join (cost=70375.50..1603795.30 rows=6802720 width=355)
Hash Cond: ("outer".fsa_id = "inner".fsa_id)
-> Seq Scan on growers (cost=0.00..672373.20 rows=6802720 width=351)
-> Hash (cost=46249.20..46249.20 rows=1966920 width=44)
-> Seq Scan on id2 (cost=0.00..46249.20 rows=1966920 width=44)
(5 rows)