Since MediaWiki 1.5 you can add
$wgReadOnly to your "LocalSettings.php" file and set this to a string describing the reason for read-only mode.
(Usual wiki markup is allowed in this string.)
This will prevent any editing on the wiki.
$wgReadOnly = "We are upgrading MediaWiki, please be patient. This wiki will be back in a few hours.";
$wgReadOnlyFile in the "LocalSettings.php" file to the name of a file, then put the reason for the lock inside the file.
The contents of the file will be displayed to anyone trying to edit the wiki.
Note that the read-only mode does not prevent all writes to the database. If you need to prevent all writes, you need to do this on the database itself (e.g. revoke UPDATE, INSERT, DELETE from the wiki user, or put MySQL into read-only mode).
$wgReadOnlyFile, so make sure the file permissions are set appropriately.