[AnonNo15]

You can do it right now by embedding such message in Bitcoin blockchain. It will give you tamper-proof and timestamped message.

[rsync]

A bit easier is to simply include a news headline in the signed message:

http://www.rsync.net/resources/notices/canary.txt

It's not as sophisticated as the blockchain, etc., but about as good - if you pick sensitive pieces of financial news it's clear you could not have known about them in advance. Or sports results.