expiration dates on cryptography

The discussion between Mr. May and Mr. Shields concerning time-release cryptograhy raised an interesting question in my mind. Given that trust is often of an ephemeral nature, it would be quite useful to set time limits on secrets. Would it be possible to cryptographically protect a secret such that it could not be decrypted after a certain time? I suspect that the laws of thermodynamics might prohibit this in classical cryptography because as a message expired the amount of entropy would decrease. Quantum cryptography might work, but that will be science fiction for some time to come. Has anyone either shown how to do this or proven it impossible?