If you add "WITH VALUES" to your ALTER TABLE ADD statement, it will put the default value in regardless of the NULL/NOT NULL setting...

USE tempdb;

GO

CREATE TABLE test (c INT NOT NULL);

GO

INSERT INTO dbo.test

(c)

VALUES

(0 -- c - int

);

GO

ALTER TABLE dbo.test

ADD create_date SMALLDATETIME NULL DEFAULT '1900-01-01' WITH VALUES;

GO

SELECT

*

FROM

dbo.test AS T;

GO

DROP TABLE dbo.test;