Why is default value not working on insert?

I have the following table:

CREATE TABLE code_source
(
csn_src int4 NOT NULL,
csn_type varchar(8) NOT NULL,
cs_code varchar(15) NOT NULL,
cs_desc_short varchar(30),
cs_desc_long text,
cs_remarks varchar(20),
cs_work_flag char(1),
cs_status char(1),
cs_manual_key bool NOT NULL DEFAULT false,
cs_create timestamp NOT NULL DEFAULT now(),
cs_live date NOT NULL,
cs_last_mod timestamp,
cs_expire date,
cs_last_mod_user varchar(12),
CONSTRAINT code_source_pkey PRIMARY KEY (csn_src, csn_type, cs_code),
CONSTRAINT code_source_fkey FOREIGN KEY (csn_src, csn_type)
REFERENCES code_source_name (csn_src, csn_type) MATCH SIMPLE
ON UPDATE CASCADE ON DELETE RESTRICT
)
WITHOUT OIDS;

As you can see, cs_create is set to not null with a default value of now().

However, when I run the following insert, it errors stating cs_create can
not be null. Why is the default not working?

insert into code_source (csn_src, csn_type, cs_code, cs_desc_short,
cs_desc_long, cs_remarks, cs_work_flag, cs_status, cs_manual_key, cs_create,
cs_live, cs_last_mod, cs_expire, cs_last_mod_user)
values ('132', 'CODE', '49', 'Some Code', null, 'NEWCODE', null, null,
false, to_date(null,'yyyymmdd'), to_date('19000101','yyyymmdd'),
to_date('20040318','yyyymmdd'), to_date('99991231','yyyymmdd'),
'MSBIUSERID');
ERROR: null value in column "cs_create" violates not-null constraint

The reason for the null being passed to to_date is this is on of almot 90k
lines of data we are trying to load, and the script was built to generate
this code. Since to_date(null,'yyyymmdd') returns a null, why is the
default not working?

Chris

PG 8.1.3
RH AS 4