Even if he didn't he should know that everyone here will want him to sign a message. He has a PGP key and several Bitcoin addresses that are known to be owned by him. Just by reading any of the "I am Satoshi" threads around and the posts in this thread and other threads as well as replies to the mailing list, Satoshi should know that in order to be taken seriously as Satoshi, he must sign something saying that that was him.