/magento/app/code/core/Mage/Adminhtml/Model/System/Config/Backend/Admin/Observer.php

<?php
/**
 * Magento
 *
 * NOTICE OF LICENSE
 *
 * This source file is subject to the Open Software License (OSL 3.0)
 * that is bundled with this package in the file LICENSE.txt.
 * It is also available through the world-wide-web at this URL:
 * http://opensource.org/licenses/osl-3.0.php
 * If you did not receive a copy of the license and are unable to
 * obtain it through the world-wide-web, please send an email
 * to license@magentocommerce.com so we can send you a copy immediately.
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade Magento to newer
 * versions in the future. If you wish to customize Magento for your
 * needs please refer to http://www.magentocommerce.com for more information.
 *
 * @category    Mage
 * @package     Mage_Adminhtml
 * @copyright   Copyright (c) 2012 Magento Inc. (http://www.magentocommerce.com)
 * @license     http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 */

class Mage_Adminhtml_Model_System_Config_Backend_Admin_Observer
{
    /**
     * Log out user and redirect him to new admin custom url
     *
     * @param Varien_Event_Observer $observer
     */
    public function afterCustomUrlChanged($observer)
    {
        if (is_null(Mage::registry('custom_admin_path_redirect'))) {
            return;
        }

        /** @var $adminSession Mage_Admin_Model_Session */
        $adminSession = Mage::getSingleton('admin/session');
        $adminSession->unsetAll();
        $adminSession->getCookie()->delete($adminSession->getSessionName());

        $route = ((bool)(string)Mage::getConfig()->getNode(Mage_Adminhtml_Helper_Data::XML_PATH_USE_CUSTOM_ADMIN_PATH))
            ? Mage::getConfig()->getNode(Mage_Adminhtml_Helper_Data::XML_PATH_CUSTOM_ADMIN_PATH)
            : Mage::getConfig()->getNode(Mage_Adminhtml_Helper_Data::XML_PATH_ADMINHTML_ROUTER_FRONTNAME);

        Mage::app()->getResponse()
            ->setRedirect(Mage::getBaseUrl() . $route)
            ->sendResponse();
        exit(0);
    }
}