I recently needed to create a rule that notifies users of a certain role when their comment is published. The rule generates a custom email to send them. Much to my surprise, there is not a default Rule condition for "Comment is Published". I found this to be interesting as there is one for "Content is Published". Fortunately the Rules API is flexible and fairly well documented. I had to create a custom 1-off module to define this condition but the code itself was very simple

/**
* Implementation of hook_rules_condition_info().
*/
function mymodule_rules_condition_info() {
  return array(
    'mymodule_rules_condition_comment_published' =----> array(
      'group' => 'comments',
      'label' => t('Comment is published'),
      'arguments' => array(
        'comment' => array(
            'type' => 'comment', 
            'label' => t('updated comment')
        ),
        'comment_unchanged' => array(
            'type' => 'comment', 
            'label' => t('unchanged comment'), 
            'handler' => 'rules_events_entity_unchanged'
        ),
      ),
      'module' => 'mymodule',
    ),
  );
}

/*
* Condition: Comment is published 
*/
function mymodule_rules_condition_comment_published($comment, $comment_unchanged, $settings) {
  if($comment_unchanged->status == 0 && $comment->status == 1) {
      return true;
  }
  else {
      return false;
  }
}

The code is pretty self-explanatory. It defines the role using the hook_rules_condition_info() hook, then compares the (saved) comment and the unchanged_comment. Below is a screenshot of the entire configuration of my rule: