Your database can be automatically saved and archived every hour and/or every day, and backups can be emailed to the address you specify. You can also restore the data to your WP DB or an external DB, which makes copying your database to another server and easy task.
Updated March-13th

EZ Backup

Keep your database safe with scheduled backups. Multiple option for off-site backups also available.

postpostmodern

Warning! In developent, not ready for production use. Should be considered alpha at best. 
Tested with WordPress MU 2.9.2 – WordPress 3.1.3 More testing to come. 
Requires PHP 5, as it should.

EZ Backup vs back data ass up