expiration dates on cryptography

[John Curtis <jbell@capecod.net>]

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?