[gherkin0]

Didn't Satoshi also sign his earliest messages with a PGP key? If this guy's really Satoshi, couldn't he just sign a new message stating so with that same private key?

[aab0]

No. Satoshi signed nothing with that PGP key that we know. Its purpose seems to have been for other people emailing him.

[jessaustin]

Even so, one who actually possessed that private key could sign a new message with it, and we could use the public key we know to verify that signature.