[nitrogen]

It would be really interesting to see PGP files that were time-sensitive, and used passwords that only worked within X time. Does anyone know if something like that has been done?

I'm not a cryptographer, but it seems to me like something of this nature is impossible without maintaining control of the decryption process. You could add a timestamp to the file, but the workaround would be to change your computer's clock or rewrite the decryption software. You would have to include a cryptographically-signed timestamp from a trusted time server in the en/decryption process. Once that signed timestamp is obtained, though, it could be distributed along with the password and a modified application that uses the stored timestamp instead of a live one from the server.

My knowledge comes from reading about failed DRM schemes and the comments of tptacek and cpercival, so I can only point out things that wouldn't work, not what will.