WordPress natively allows us to wrap comments in <div>’s or <li>’s but not HTML5 <article> elements. If you want to do this, add an ‘end-callback’ param to the wp_list_comments function call and set ‘article’ as the style (in comments.php):

    wp_list_comments(array('callback' => 'twentyten_comment',
                           'style' => 'article',
                           'end-callback' => 'twentyten_comment_end'));

…then in functions.php, add the callback function:

    if(!function_exists('twentyten_comment_end')) :
    /**
     * Allow HTML5 <article> (or anything else) for comment container.
     */
    function twentyten_comment_end($comment, $args, $depth) {

        switch($args['style']) {
            case 'ol':
            case 'ul':
            case '':
                echo "</li>\n";
            break;
            default:
                echo '</' . $args['style'] . ">\n";
        }
    }
    endif;