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