Highlighting the current users comment

assumes that your theme is using comment_class();

example (to be added to functions.php of your theme):

add_filter( 'comment_class', 'comment_class_logged_in_user' );

function comment_class_logged_in_user( $classes ) {
    global $comment;
    if ( $comment->user_id > 0 && is_user_logged_in() ) {
        global $current_user; get_currentuserinfo();
        $logged_in_user = $current_user->ID;
        if( $comment->user_id == $logged_in_user ) $classes[] = 'comment-author-logged-in';
    }
return $classes;
}

requires formatting of the css class:

.comment-author-logged-in { }

I recently posted a plugin version in my site.