usually, each user's MySQL database is backed up nightly to an external host, in case of disk failure or other loss of data. currently, this is NOT being done, because of the following MySQL bug:
http://bugs.mysql.com/bug.php?id=24070
therefore, you should take your own backups of any important data. i will inform the list when this bug is resolved.
- river.
toolserver-l@lists.wikimedia.org