Programmatically turn off body summary
I placed this code directly after my call to node_add_body_field('my_node_type').

  $field_instance = field_info_instance('node', 'body', 'my_node_type');
  $field_instance['settings']['display_summary'] = FALSE;
  field_update_instance($field_instance);


References


node_add_body_field()
field_info_instance()
field_update_instance()