why am I getting a seq scan on this query?

I'm expecting this to do an indexed scan... any clue why it's not?  This is
with PG 7.4.

Thanks!!

planb=# explain select id,shotname from df_files where showid=30014515::bigint;
                             QUERY PLAN
-------------------------------------------------------------------
  Seq Scan on df_files  (cost=0.00..791035.45 rows=540370 width=22)
    Filter: (showid = 30014515::bigint)
(2 rows)



planb=# \d df_files;
                Table "public.df_files"
   Column   |            Type             | Modifiers
-----------+-----------------------------+-----------
  id        | bigint                      | not null
  showid    | bigint                      | not null
  shotname  | character varying(256)      | not null
  elemname  | character varying(256)      | not null
  frameno   | character varying(12)       | not null
  ext       | character varying(12)       | not null
  filename  | character varying(256)      | not null
  filesize  | bigint                      |
  locked    | boolean                     |
  timestamp | timestamp without time zone |
Indexes:
     "df_files_pkey" primary key, btree (id)
     "df_files_elemname" btree (elemname)
     "df_files_ext" btree (ext)
     "df_files_filename" btree (filename)
     "df_files_frameno" btree (frameno)
     "df_files_shotname" btree (shotname)
     "df_files_show" btree (showid)
     "df_files_showid" btree (showid)

planb=# select count(*) from df_files where showid=30014515::bigint;
  count
--------
  528362
(1 row)

Time: 420598.071 ms
planb=# select count(*) from df_files;
   count
----------
  24415513
(1 row)

Time: 306554.085 ms