One of those little Drupal annoyances this, there is now easy way to alter a comment (using hook_comment) before the comment is written to the DB. Hook comment is invoked during the validation phase, with $op set to "validate", and the comment array is passed by reference - but unfortunately any changes made to the comment are not passed on to the comment save function.
A quick and slightly dirty way around this is to make use of the global $form_values array and manipulate the comment directly in there - this little example will set an empty comment title to...
