How to define a timestamp field using drupal6 schema
In short, you cannot do this because the schema API does not support the TIMESTAMP type.


  The MySQL implementation of the Schema API implements the datetime field using MySQL's DATETIME type. That type does not support automatic initializiation/update to the current time.
  
  MySQL's type TIMESTAMP supports automatic initializiation/update to the current time. Switching from DATETIME to TIMESTAMP is not an option, though:
  
  TIMESTAMP behaves differently in MySQL versions 4.1, 4.1.2, 4.1.3, 4.1.6
  The first TIMESTAMP column in a table behaves different than other TIMESTAMP columns in that table.