I'd like to suggest a wallet feature. If I'm understanding correctly, the wallet has 100 pre-generated addresses that are being used every time I send coins or create a receive address myself. If I back up my wallet when I have used 90 addresses, then send coins 20 times, and accidentally format my drive and lose the wallet, my backup wallet is not valid anymore. If this is true and how it works, I suggest that the wallet displays a warning when it has generated new addresses that tell the user that the old backup is not valid anymore and that the user should make a new backup immediately.
I had given some thought to this as well. To take this one step further, this limitation should be removed. I'm not sure what the mechanics are, but having your backup invalidated after a set amount of transactions occurs sounds counter-intuitive. At the very least, if this limitation can't be removed, the limit should be significantly increased.
The Armory wallet for Bitcoin only has to be backed up once. That would be ideal!