Actually, handy, the default theme IS hardcoded into WordPress:
wp-includes/comment-template.php:
define('COMMENTS_TEMPLATE', true);
$include = apply_filters('comments_template', TEMPLATEPATH . $file );
if ( file_exists( $include ) )
require( $include );
else
require( ABSPATH . 'wp-content/themes/default/comments.php');
wp-includes/general-template.php:
function get_header() {
do_action( 'get_header' );
if ( file_exists( TEMPLATEPATH . '/header.php') )
load_template( TEMPLATEPATH . '/header.php');
else
load_template( ABSPATH . 'wp-content/themes/default/header.php');
}
function get_footer() {
do_action( 'get_footer' );
if ( file_exists( TEMPLATEPATH . '/footer.php') )
load_template( TEMPLATEPATH . '/footer.php');
else
load_template( ABSPATH . 'wp-content/themes/default/footer.php');
}
function get_sidebar() {
if ( file_exists( TEMPLATEPATH . '/sidebar.php') )
load_template( TEMPLATEPATH . '/sidebar.php');
else
load_template( ABSPATH . 'wp-content/themes/default/sidebar.php');
}