Query not using index, please explain.

Greetings,

I have a real simple table with a timestamp field. The timestamp field has
an index on it. But, the index does not seem to be taken into account for
selects that return rows:

pglog=# explain select time_stamp from history_entries where time_stamp <
'03-01-2000';
NOTICE: QUERY PLAN:

Index Scan using hist_entries_timestamp on
history_entries (cost=0.00..12810.36 rows=3246 width=8)

EXPLAIN
pglog=# explain select time_stamp from history_entries where time_stamp <
'04-01-2000';
NOTICE: QUERY PLAN:

Seq Scan on history_entries (cost=0.00..160289.71 rows=138215 width=8)

EXPLAIN
pglog=# set enable_seqscan to off;
SET VARIABLE
pglog=# explain select time_stamp from history_entries where time_stamp <
'04-01-2000';
NOTICE: QUERY PLAN:

Index Scan using hist_entries_timestamp on
history_entries (cost=0.00..368241.51 rows=138215 width=8)

EXPLAIN
pglog=# set enable_seqscan to on;
SET VARIABLE
pglog=#

The query where the time_stamp < '03-01-2000' does not return any rows, the
04-01-2000 date does return rows. When I disable seqscan the query is
almost instant, but with it on, it takes about 3 or 4 minutes. Why can't
the query planner use the index in the later case?

Thanks,
Matthew