problem inserting with sequence

I have been trying to figure out why I can't insert
into a table and I think it has something to do with
the sequnce.

I am able to use able to properly insert into the
table using the the shell, but I am not able to insert
using a php script:

INSERT INTO requests (time_stamp, req_num,
recommended_by) VALUES (now(),
nextval('requests_req_num_seq'), 'foo');

Here is the schema I'm using:
DROP SEQUENCE requests_req_num_seq;
DROP TABLE requests;

CREATE SEQUENCE requests_req_num_seq INCREMENT BY 1
START WITH 1000;

CREATE TABLE requests (
time_stamp timestamp PRIMARY KEY DEFAULT 'now',
req_num integer DEFAULT
nextval('acq_requests_req_num_seq') NOT NULL,
recommended_by varchar(35) NOT NULL
);
GRANT ALL ON requests TO wwwrun, postgres;

If I reconfigure the schema by removing the sequence
from the table, I am able to insert into the table
using both the shell and php script:
DROP SEQUENCE requests_req_num_seq;
DROP TABLE requests;

CREATE SEQUENCE requests_req_num_seq INCREMENT BY 1
START WITH 1000;

CREATE TABLE requests (
time_stamp timestamp PRIMARY KEY DEFAULT 'now',
recommended_by varchar(35) NOT NULL
);

INSERT INTO requests (time_stamp, recommended_by)
VALUES (now(), 'foo');

I'm a little stumped on this one. I'm running
PostgreSQL 7.4.8 on i686-pc-linux-gnu, compiled by GCC
gcc (GCC) 3.3.3 (SuSE Linux).

Thanks in advance for any help or advice.

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