I ran into issues when trying to use drupal's hook_form_alter to change some attributes of cck fields in a node form. It appears that depending on the weight of the module the form_alter is in, it might get called before the actual form element has been processed.
To make sure, you can set a function to be called after the form has been properly created, but before rendering.
In your form alter, add the after build key to the form field's array:
$form['field_course_status']['#after_build'] = 'course_disable_status_field';