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