Magento 1.4 introduced a method for quickly enabling maintenance mode on a running store. To enabled maintenance mode simply create a file called maintenance.flag file in your Magento root.
The maintenance mode template is found in the errors/default/503.phtml. Integration with the “Allowed IPs” in the developer section of the configuration section would be nice but, of course, there is a way you can hack it. Without this hack maintenance mode is enabled for everyone, even if you have an admin session.