How to let a single post have its own domain name

This code will allow you to set a custom meta value, and if the domain name (or subdomain if you edit the code) matches it, the query will be changed to match that post. The page template will only be used for that request, not for requests via the “normal” URL.

This does not change links on that page: should they go to the “normal” site or stay in the subdomain?

How you solve this on the DNS side is probably a Server Fault question.

define( 'WPSE4558_STANDARD_SERVER', 'www.example.com' );
define( 'WPSE4558_META_KEY', 'domainname' );

add_filter( 'request', 'wpse4558_request' );
function wpse4558_request( $query_vars )
{
    $query_vars['is_subdomain_request'] = false;
    if ( WPSE4558_STANDARD_SERVER != $_SERVER['SERVER_NAME'] ) {
        $query_vars['meta_key'] = WPSE4558_META_KEY;
        // This can also be just the subdomain, if you edit it
        $query_vars['meta_value'] = $_SERVER['SERVER_NAME'];
        $query_vars['is_subdomain_request'] = true;

    }
    return $query_vars;
}

add_action( 'parse_query', 'wpse4558_parse_query' );
function wpse4558_parse_query( &$wp_query )
{
    if ( $wp_query->get( 'is_subdomain_request' ) ) {
        $wp_query->is_home = false;
        $wp_query->is_page = true;
        $wp_query->is_singular = true;
    }
}

add_filter( 'page_template', 'wpse4558_page_template' );
function wpse4558_page_template( $template )
{
    global $wp_query;
    $id = $wp_query->get_queried_object_id();
    if( ! $wp_query->get( 'is_subdomain_request' ) && get_post_meta( $id, WPSE4558_META_KEY ) ) {
        // This is a page that has a subdomain attached, but the current request is not via that subdomain
        // So use the normal template hierarchy, ignore the page template
        $templates = array();
        $pagename = $wp_query->get_queried_object()->post_name;
        if ( $pagename ) {
            $templates[] = "page-$pagename.php";
        }
        if ( $id ) {
            $templates[] = "page-$id.php";
        }
        $templates[] = "page.php";
        $template = locate_template( $templates );
    }
    return $template;
}