Note: I am not a lawyer and cannot give legal advice. See a lawyer if you need legal advice.
Many people would like to believe that the Fifth Amendment to the U.S. constitution lets them keep their password or pass phrase a secret. "No person … shall be compelled in any criminal case to be a witness against himself" But the law is not that simple. On several occasions, the U. S. government has gotten courts to order defendants to decrypt their hard drive, rather than ask for the password itself.
The question of whether and when the U.S. government can force a criminal suspect to decrypt data has finally reached a higher court. On February 23, 2012, The U.S. Court of Appeals for the Eleventh Circuit issued a ruling in U.S. v. John Doe that limits the governments ability to force someone to decrypt their hard drives. As I read it, the ruling says the government can only demand a hard drive be decrypted if it already has some specific knowledge about the files contained on that drive, so that the act of producing them would not constitute testimony that the files exist.
In January 2012, a federal district judge in Colorado ordered a criminal defendant to decrypt her laptop's hard drive. "I find and conclude that the Fifth Amendment is not implicated by requiring production of the unencrypted contents of the Toshiba Satellite M305 laptop computer." https://www.eff.org/cases/us-v-fricosu http://news.cnet.com/8301-31921_3-57364330-281/judge-americans-can-be-forced-to-decrypt-their-laptops/
The appeals court distinguished its case from Colorado case (which is in a different circuit) because there the government had wiretaps which mentioned data on the defendant's laptop. The appeals court decision is worth reading if you're interested in this subject. It's at https://www.eff.org/sites/default/files/filenode/OpinionDoe22312.pdf