Uh. So I just read some case law, and though I am not a lawyer, the current SCOTUS stance is that this sort of compelled speech doesn't constitute a testimony and thus *isn't* protected.
http://supreme.justia.com/cases/federal/us/487/201/
Sorry, guys, but everyone who has said case law says this speech is protected is wrong. It actually looks like the Judge in Michigan mis-read the two cases he cites, which is very odd. Doe v. US explicitly states that "testimony" has to relate to a statement of fact. US v. Hubbel cites Doe v. US because the case relates to actual factual information compelled from the suspect. This meets the minimum definition of testimony which a password does not. Subsequent cases have of course, followed this approach.
The most relevant case law is Boucher II (the end result of the vermont case turtle mentions).