Read-Once Messages?

Has there been any work done on messages that can be read a single time, preferably only by a designated recipient, and is not amenable to being captured as it is "played"? I know that Gibson's poem _Agrippa_ had some sort of self-destruct feature built into it, but I don't know what mechanism was used to implement this. Any pointers or suggestions would be appreciated... -- Lefty (lefty@apple.com) C:.M:.C:., D:.O:.D:.