Cross-datatype Comparisons and Indexes

Since the current stable version of postgres (7.4.x) doesn't allow
cross-datatype comparisons of indexes, is it always necessary to cast
my application data explicitly in order for an index to be used, even
among the integer types?

E.g., If I have a table with a bigint primary key and application data
compared against that primary key, must I always explicitly cast the
application data to bigint if I want postgres to use the index?

SELECT * FROM some_table WHERE primary_key_column = 42

vs.

SELECT * FROM some_table WHERE primary_key_column = 42::int8

There's no way to tell postgres to treat incoming data implicitly as
the type of the column, right?

Thanks!

-tfo