Mining involves generating hashes, which apparently GPU hashes are good at. I'm not sure what algorithm is used for encrypting your private keys, but I guess the first thing to figure out is if a GPU would be any good at that algorithm anyhow.
Then assuming that it is, I would think you could pull an encrypted private key from the wallet and repeatedly attempt to decrypt it, looking to see if the result is a valid private key without having to go back to bitcoind every time.
I will look into this, who knows if bitcoins keeps going up cracking wallets with lost passwords will be worth greater and greater investment. I was considering writing a short story about it.