Mike McNally writes:
* The tricky case is one in which the key is actually a phrase like "I am a drug dealer and I did the crime". I'm not exactly sure how this would be used in court; maybe something like "I refuse to reveal that information on the grounds that the key itself is in the form of a statement which could be construed to be incrimintating." Of course, I don't know why you couldn't always say that...
If Mr. Godwin isn't too bothered by the re-emergence of this thread, he may choose to share with us further thoughts on the topic raised in the second bullet. My guess is that such a situation will have to be tested in court, and I can only hope that it gets tested with the assistance of some very skilled counsel.
I am now convinced that even if the key were a statement of the form "I did crime X", it could still be compelled, because the actual statement is not testimonial, and would not be used as evidence (e.g., as an admission). --Mike