i would love this feature
even better: show a warning message if there is a privkey going to be used which is not contained in the last backup.
This. From a usability standpoint having a counter is just another stats, number, widget which adds screen clutter and confusion. However storing the last key included in the backup and then warning the user when trying to use (either by change address or new address button) a key not included in the last backup would be useful.
This operation will send bitcoins to a key not included in your last backup. Funds may be lost if wallet is lost or corrupted.
It is strongly recommended you create a backup of your wallet before making this transaction. Do you want to backup your wallet now?
[Yes] [No (Loss of funds possible)] [Backup after this transaction is completed]
[ ] Do not warn me about the potential for key loss again
Hell maybe in "noob mode" the wallet simply wouldn't allow you to do anything until you make a backup.
The backup of your wallet is out of date. This transaction can not be recovered from your existing backup. Funds may be lost if the wallet becomes lost or corrupt.
You must make a backup of your wallet before continuing with this transaction.[Make Backup Now]
If the second method is used I would include a "nag screen" (can be disabled in options) which starts when there are 10 or less backed up keys in the keypool. Granted it isn't "foolproof"; a user can click make backup and then not safely store it. User can make backups outside the GUI and get false warnings but it would provide "pretty good protection".
Another easy precaution would be to simply increase the default size of the keypool. The default is currently 100. Even a casual user may use 1 or 2 addresses per day. That means a backup is only good for roughly 60 days worth of addresses. More active users (satoshi dice) could obsolete their backup in a matter of days. Pretty easy for an inexperienced user to be running on a wallet no longer protected by a backup and not know it. Given how tiny keys are I think making the default size 500 or even 1000 keys would be very useful. Even 1,000 keys in a keypool makes the initially wallet only ~300KB. Compared to a block chain in GB trying to save a couple hundred KB and putting funds at risk seems silly.
Advanced users who need ultra-small wallet sizes can modify the keypool to be smaller (even a keypool of zero) just like they could disable the backup warnings.