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Some software like Symantec's Ghost 9 will write backups directly to your DVD for you. However it doesn't seem like this is a job that can be directly scheduled with the software. (I'm sure a batch file of some sort could work around this limitation.)
For a totally automated solution you would need a disk changer to swap out backups. Maybe if you could load DVD-RW discs on a carousel of some sort you could have a completely automatic system.