As I understand, and to my experience, in Armory you can take a single backup, and the program will be able to regenerate all addresses in a wallet based upon this one backup. It means that you can take one backup when you create the wallet, and that's all you need. It will use the information from the first address as a seed to generate the next, so you don't risk loosing the other private keys.
Is there something similar implemented in the Qt client? Or do I have to back it up every time I create a new address?
What information is used as seed when I generate a new address in the Qt client?
BIP 32 was developed by the Bitcoin-Qt devs, and will hopefully make it's way into Bitcoin-Qt in 0.8. But it's looking like it'll be a later release. This will provide multi-chain deterministic addresses (create multiple wallets built from one seed). I plan to implement the same thing in Armory so it will be compatible.
But at the moment, you are correct: this cannot be done in Bitcoin-Qt, and it's a real shame because requiring-regular-backups-at-the-cost-of-losing-your-money is a real problem in the world of end-user software. It's never worth dealing with backups until it's too late, i.e. "ehhh, I'll set it up next week." Plus, that means that your backup solution can't be extremely secure, because it still needs to be convenient since you're doing it multiple times. At least with a deterministic wallet, you can backup once, drive to the bank and put it in your safe-deposit box. Once.
Personally, I think this is the number one reason to use these alt clients, because the ability to backup once,
forever is such a powerful feature.
