Giving a defendant limited immunity in terms of forcing them to turn over the passphrase can lead to a conviction. That's because the fellow technically isn't being convicted based on his passphrase; he's being convicted for what it unlocks. Isn't the law grand?

This is the sort of sophistry that kills civil liberties... The idea that the government can compell you to divulge an encryption key without violating your 5th amendment rights by immunizing you from prosecution due to the contents of the encryption key, but not the contents of files encrypted with that key.