I think it was Schneier's blog that argued or highlighted that it's simultaneously a Fourth and Fifth Amendment issue. On Fourth, they're trying to search through your digital property. At same time, getting your password requires you to speak something that might get you charged for a crime. If it's encrypted messages, those are your speech as well in digital form.

Different judges are looking at it in different ways. I say assume they can search it or hold you in contempt for the password if they intercept or find it. Then, you shift the goal post to them not being able to do that.