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Consider that the cyphertext is a program for an abstract machine called the cyphercracker which returns TRUE if a message is encoded otherwise FALSE. Such a system for determining message-ness could take an arbitrary amount of cpu time and no amount of static analysis could determine the return value quicker.

Nope. Such a system will take no more than O(2^n) time, where n is the number of bits in the key. You can never do worse than brute-force. Now, you still might not be able to determine if a message is encoded, since maybe I was just encoding true random noise from a radioactive source. And you might have false positives, too, esp. with one-time pads. But it will always halt. The failure modes have nothing to do with the halting problem, they have to do with the fact that is-encoded(message) cannot be formally defined. Marc