PostgreSQL doesn't use indexes even is enable_seqscan = off

I have a severe problem with PostgreSQL 7.2.1.
I have a table containing 500mio records (for testing purposes).

I have indexed the table:

CREATE UNIQUE INDEX idx_one_id ON one(id);
CREATE INDEX idx_one_xmod ON one(xmod);

The index was created properly but somehow it isn't used:

cluster=# \d one
          Table "one"
  Column |  Type   | Modifiers
--------+---------+-----------
  id     | bigint  |
  even   | boolean |
  xmod   | integer |
Indexes: idx_one_xmod
Unique keys: idx_one_id

cluster=# SET enable_seqscan TO off;
SET VARIABLE
cluster=# SELECT * FROM one WHERE id=300000;
Cancel request sent
ERROR:  Query was cancelled.
cluster=# EXPLAIN SELECT * FROM one WHERE id=300000;
NOTICE:  QUERY PLAN:

Seq Scan on one  (cost=100000000.00..109434714.00 rows=1 width=13)

EXPLAIN
cluster=# SELECT version();
                            version
-------------------------------------------------------------
  PostgreSQL 7.2.1 on i686-pc-linux-gnu, compiled by GCC 2.96
(1 row)

When sequential scans are turned off PostgreSQL should use an index but
it doesn't.
Is it a bug? Have I done something one?
Did anybody face a similar problem?

    Hans