Null values in Postgresql date comparison

test=# begin;
BEGIN
test=# create temp table test(test_date date) on commit drop;
CREATE TABLE
test=# insert into test(test_date) values (null);
INSERT 0 1
test=# insert into test(test_date) values (current_date - interval '1 day');
INSERT 0 1
test=# insert into test(test_date) values (null);
INSERT 0 1

test=# select case when test_date < current_date then 'less' else 'nope' end, coalesce( test_date::varchar, 'NULL') from test;

 case |  coalesce  

------+------------

 nope | NULL

 less | 2012-03-20

 nope | NULL

(3 rows)