This is a very cool and nicely configurable plugin.
One bug fix needed:
site.php, line 95:
<?php bloginfo('url') ?>
<?php echo site_url(); ?>
Otherwise, the countdown timer won't work, if your Site Address isn't at the root of your site (i.e. if need to use /wp/wp-includes...).
And, you can't fully rely on this plugin for any serious WordPress maintenance (such as a manual upgrade), because the plugin is part of WordPress and thus requires WordPress to be in a functional state. An all-purpose solution would involve an .htaccess rule that temporarily redirects to a static HTML files, thus not depending on WP at all.
But, if you have to do it within WordPress, this plugin is great!