This filter is called just before the comment is added to the post. See this for more information about the data in the $comment array.
add_filter('postie_comment_before', 'my_postie_comment_before');
function my_postie_comment_before($comment) {
$post = get_post($comment['comment_post_ID']);
$title = $post->post_title;
echo "postie_comment_before called<br>\n";
return $comment;
}