Q: Table scans on set difference

What's happening here?

I have two tables, encounter_properties_table with about 100000 rows and
xfiles with about 500000 rows. The structures of these tables is as follows:

       Table "public.encounter_properties_table"
     Column     |           Type           | Modifiers
----------------+--------------------------+-----------
 timestamp      | timestamp with time zone | not null
 practice_id    | integer                  | not null
 patient_id     | bigint                   | not null
 properties     | text                     |
 modified_by    | bigint                   | not null
 client_version | integer                  |
 file_name      | character varying(255)   |
Indexes:
    "encounter_properties_table_pkey" primary key, btree (patient_id)
    "fn_ix" btree (file_name)

and

             Table "public.xfiles"
  Column  |          Type          | Modifiers
----------+------------------------+-----------
 filename | character varying(100) | not null
Indexes:
    "xfiles_ix1" btree (filename)

The following query shows that PostgreSQL 7.4 is doing table scans on both
tables:

explain select file_name from encounter_properties_table where file_name not
in (select filename from xfiles);
                                       QUERY PLAN
----------------------------------------------------------------------------
------------
 Seq Scan on encounter_properties_table  (cost=0.00..1030610198.10
rows=85828 width=58)
   Filter: (NOT (subplan))
   SubPlan
     ->  Seq Scan on xfiles  (cost=0.00..10755.44 rows=500944 width=59)
(4 rows)

I ran vacumm analyze on both tables.

We aborted this query when it had not finished after 4 hours.

We ran the same query on SQLServer 2005 with the same data and it took under
one second to finish.

Any ideas?