define( 'WPSE4558_STANDARD_SERVER', 'hello-world.apkgal.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;
}