http://dev.mysql.com/doc/refman/5.0/en/timestamp-initialization.html
In addition, you can initialize or update any TIMESTAMP column to the current date and time by assigning it a NULL value, unless it has been defined with the NULL attribute to permit NULL values.
In order to allow a TIMESTAMP to be nullable, create it using the NULL attribute, or alter the table and add the NULL attribute. In a create statement, it would resemble.
CREATE TABLE t1 (tsvalue TIMESTAMP NULL, ... );
Inserting a NULL value into a TIMESTAMP field with the NULL attribute set will result in the field being set to NULL instead of to NOW().