We were just updating a client site and needed to put up a temporary “Maintenance” page up while we did some major changes to their site. This little snippet should be included in your WordPress theme’s functions.php file.

add_action('wp', 'coming_soon');
function coming_soon() {
if (current_user_can('manage_options')) return;
wp_die('We are quickly performing an upgrade to our site - should be back up shortly.');
}