I am using something similar to this to include a webpage when my Wordpress wordpress plugin is triggered.

function create_wordpress_pages(){

$my_post = array();
$my_post['post_title'] = 'My post';
$my_post['post_content'] = 'This is my post.';
$my_post['post_status'] = 'publish';
$my_post['post_author'] = 1;
$my_post['post_type'] = 'page';

wp_insert_post( $my_post );
register_activation_hook(__FILE__, 'create_wordpress_pages');

This really is fine (and dandy) but I'd rather not re-create this site each time this wordpress plugin is deactivated. How do you determine if the page is available OR unpublish on wordpress plugin deactivation?

You should check when the page is available using Wordpress_Query. Simple as that. To remove on deactivation, use register_deactivation_hook.