I saw it, was about to comment but thought my concept with its benefits is too far off your idea so I decided to do a new thread. My idea involves no modification on the network layer, no obscure security with mouse coordinates, etc.
If you just drop a few digits from the private keys, it's trivial to crack, as an exhaustive search will retrieve them easily from a leaked wallet.dat file.
If you drop a lot of digits from the private keys, it's not better than a passphrase-encrypted wallet (as will be supported by 0.4.0), and a whole lot less user-friendly.
As described in the OP the user could decide how many digits to use. I think it is a pain to handle several wallet files today but improvements there would at least allow me to have small wallets with a password I can remember and big wallets with a password i keep in a save. else having to type in "the" password would expose all my wallet to the attacker's trojan.
I'm curious how the new client will turn out to be but I doubt that I will feel any saver opening my big wallets with it than I feel now.
Lastly both concepts could be combined.