BE can be locked by maintenance. For that a file typo3conf/LOCK_BACKEND needs to be created – either manually, or using the CLI script lowlevel_admin. CLI scripts will still have access to the system, and could be used for maintenance during that time. More info: http://forge.typo3.org/repositories/diff/typo3v4-core?rev=4901 RFC: #10322: Send Notification if BE is locked http://bugs.typo3.org/view.php?id=10322