Virtucon writes "U.S. Magistrate William Callahan Jr. of Wisconsin has ruled in favor of the accused in that he should not have to decrypt his storage device. The U.S. Government had sought to compel Feldman to provide his password to obtain access to the data. Presumably the FBI has had no success in getting the data and had sought to have the judge compel Feldman to provide the decrypted contents of what they had seized. The Judge ruled (PDF): 'This is a close call, but I conclude that Feldman's act of production, which would necessarily require his using a password of some type to decrypt the storage device, would be
tantamount to telling the government something it does not already know with "reasonably particularity" — namely, that Feldman has personal access to and control over the encrypted storage devices. Accordingly, in my opinion, Fifth Amendment protection is available to Feldman. Stated another way, ordering Feldman to decrypt the storage devices would be in violation of his Fifth Amendment right against compelled self-incrimination.'"
If the government has reasonable suspicion that you have illicit data, they can still compel you to decrypt it.