BUG: bigint and indexes ?

Is it a known bug if optimizer, that Postgres can't use indexes created on a int8 (bigint) ?
This problem exists in 7.1.3 and even in 7.2.

test=# CREATE TABLE parent_child (
test(# parent_id int8 NOT NULL,
test(# parent_type int4 NOT NULL,
test(# child_id int8 NOT NULL PRIMARY KEY,
test(# child_type int4 NOT NULL,
test(# account_id int8 NOT NULL,
test(# p_begin date,
test(# suspended int4);
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index 'parent_child_pkey' for table 'parent_child'
CREATE
test=# explain select * from parent_child where child_id=1;
NOTICE: QUERY PLAN:

Seq Scan on parent_child (cost=0.00..22.50 rows=1 width=40)

EXPLAIN
test=# create index parent_child_childtype on parent_child(child_type);
CREATE
test=# explain select * from parent_child where child_type=1;
NOTICE: QUERY PLAN:

Index Scan using parent_child_childtype on parent_child (cost=0.00..17.07 rows=5 width=40)

EXPLAIN
test=#