I agree that this is how it should start, but honestly, I don't think that it would be an ultimate proof for me. It would prove that this person was very close to Satoshi psychologically (since Satoshi entrusted this person with the keys) or literally (so this person could steal access). But not that it's necessarily Satoshi Nakamoto. Craig, for instance, might succeed in opening the wallet soon (although I doubt that), but I would rather believe that the real Satoshi died and Craig stole his ideas and money. So to sum this up, signing a message is necessary, but not sufficient proof that a person is Satoshi, at least for me.
He can also prove his identity if he just logs into his Bitcointalk forum account as we spoke about it before:
Bitcointalk account is locked. Must send valid proof that can be easily and publicly verified to unlock account.
Signed message from PGP key. Or signed messages from bitcoin addresses used in genesis block up to block 170.
It's possible Satoshi may have lost the private keys or wallets, but he'd never lose access to his PGP key. Satoshi did remind everyone, never to delete wallets. He would have kept a backup somewhere.