Theme switcher proof of concept

<?php
/*
Plugin Name: zed1.com Theme switcher
Plugin URI: http://zed1.com/
Description: A plugin that switches themes based on the URI
Version: 0.0.1
Author: Mike Little
Author URI: http://zed1.com/
License: GPL2+
*/

$z1_which_stylesheet = 'twentyeleven';
$z1_which_template = 'twentyeleven';
add_action('setup_theme', 'z1_determine_theme');
function z1_determine_theme() {
    global $z1_which_stylesheet, $z1_which_template;
    if ( isset( $_SERVER['REQUEST_URI'] ) && ( false !== strpos( $_SERVER['REQUEST_URI'], '/shop/' ) ) ) {
        $z1_which_stylesheet = 'twentyten';
        $z1_which_template = 'twentyten';
    }
}

add_filter('stylesheet', 'z1_change_the_stylesheet');
function z1_change_the_stylesheet($stylesheet) {
    global $z1_which_stylesheet;
    return $z1_which_stylesheet;
}

add_filter('template', 'z1_change_the_template');
function z1_change_the_template($template) {
    global $z1_which_template;
    return $z1_which_template;
}


?>