If I understand the topic of digital signatures correctly, you sign a message via hashing+encrypting it using your private key.
Then, if a private key is compromised by an attacker, she could sign stuff pretending she's you. This way, any organization that has the (even legal) power of asking you to decrypt your messages such to prove to a court there's no evidence of illegal action in them, has now the power of signing messages that state you did things you never did.
Is there any way for attaching a computationally-hard-to-fake timestamp (not silly cleartext meta's, course...) on a signature such that you can not only revoke a key after having been obliged to release it, but that you can also prevent them to now sign every message they want it to look you wrote?