You’ll get this error when Wordpress automatic update process, via svn or admin, fail or is incomplete. It leaves the file named ”.maintenance” on your home or root directory, with info on maintenance.
Sample content of .maintenance file:
<?php $upgrading = 1282258195; ?>
Just delete or rename that file and resume your update process, or you may want to restore your backup first and re-start the update process. Of course, check what cause the failed or incomplete update. :)