That only means you don't have to say anything to the police while you are in custody but have not yet been charged with a crime, and your lawyer is not present. It's not meant as a blanket protection against having to give testimony.
http://en.wikipedia.org/wiki/Fifth_Amendment_to_the_United_S...
"If the government gives an individual immunity, then that individual may be compelled to testify. Immunity may be "transactional immunity" or "use immunity"; in the former, the witness is immune from prosecution for offenses related to the testimony; in the latter, the witness may be prosecuted, but his testimony may not be used against him. In Kastigar v. United States, 406 U.S. 441 (1972), the Supreme Court held that the government need only grant use immunity to compel testimony. The use immunity, however, must extend not only to the testimony made by the witness, but also to all evidence derived therefrom. This scenario most commonly arises in cases related to organized crime."
The evidence is what it is, the court is there to judge it.
There have in fact been a few instances where judges have determined that providing a password does constitute giving testimony.
http://www.easterndistrictblog.com/?p=57
and older
http://mises.org/Community/blogs/crypto/archive/2007/12/18/c...
Where did you put your ex-wife's body Mr Johnson, we know you killed her? Just point at the map, you do not even have to say anything.
You could have cat pictures encrypted with passphrase A and incriminating evidence (that stands up to the best forensic analysis currently available) encrypted with passphrase B and they wouldn't know which is which, if they unlocked ALL the data, or what.
Truecrypt is AMAZING and anyone holding onto stuff that might get them into trouble (esp. w/ foreign gov'ts) should use it.