Very simple WordPress plugin that warns a user if their blog can not be indexed by search engines.

blog-not-public-admin-notice.php

<?php
/*
 * Plugin Name: Blog not public admin notice
 * Description: Because this checkbox costs real people real money
 * Version: 1.0.0
 * Author: daggerhart
 */

add_action( 'wp_ajax_dismiss_blog_not_public_notice', function(){
	update_option('blog_not_public_notice_dismissed', 1);

	if ( !$_GET['is_ajax'] ){
		wp_redirect( wp_get_referer() );
	}

	exit;
});

add_action( 'admin_notices', function(){
	$public = get_option( 'blog_public', 0 );
	$notice_dismissed = get_option( 'blog_not_public_notice_dismissed', 0 );

	if ( !$public && !$notice_dismissed )
	{
		?>
		<style>
			#blog_not_public_notice {
				float: right;
				margin: 6px;
			}
		</style>
		<script type="text/javascript">
			(function($){
			    $(document).ready(function(){
			       $('#blog_not_public_notice').click(function(event){
			           event.preventDefault();
			           var $btn = $(this);

			           $.get( $btn.attr('href'), { is_ajax: true }, function(){
			               $btn.closest('.error').fadeOut();
			           } );
			       })
			    });
			})(jQuery);
		</script>
		<div class="error">
			<a id="blog_not_public_notice" href="<?php echo esc_attr( admin_url('admin-ajax.php?action=dismiss_blog_not_public_notice') ) ?>"><?php _e( 'dismiss' ); ?></a>
			<p>
				<strong><?php _e('Site hidden from search engines') ?></strong>
				<?php _e('This website is not able to be indexed by Google or other search engines. To fix this problem click on the following link and un-check the "Search Engine Visibility" checkbox.') ?>
				<a class="button" href="<?php esc_attr( admin_url('options-reading.php') ) ?>"><?php _e('Click here to fix') ?></a>
			</p>
		</div>
		<?php
	}
} );

Screenshot:

Inspiration: