[cheald]

IANAL, so I'm unqualified to say, but the way I read the ruling was that they decided that you can't be compelled to hand over your password, but because Boucher had already unlocked the drive for law enforcement once, doing so again would not further incriminate him. So, they forced him to unlock the hard drive without disclosing the password.

As I understand it, if he had initially refused to unlock the drive, then he couldn't have been compelled to unlock it again, under grounds that it could be self-incrimination.