The constitution doesn't protect you from keeping silent on the contents of your own mind, it keeps you protected from taking the stand, in a trial, and verbally incriminating yourself.
That seems like a pretty narrowly defined interpretation, that I'm not sure is actually valid.
In any case, here are my thoughts. I've been reading some of the discussion elsewhere (ars techinca has a ton of good debate happening on this matter). One of the things that's caused me the most trouble was one remark, which I'll have to paraphrase. Somebody said that the Bill of Rights was in place to protect us all from the law, not to help us break it. As much as I'd like to, I can't refute this. It's just a clear and succinct interpretation that's almost certainly what Madison et al had in mind. Where I depart with that interpretation is how it effects things in the future, which as we all agree bears no resemblance to what they could have foreseen.
Part of living in a free society is living with risk. A happy-go-lucky schizophrenic might shoot up a political rally. Nineteen assholes might crash planes into your office. Creepy fucks might look at pictures they're not supposed to. People might commit crimes which you're obligated to let them get away with. There are reasonable things that can be done to prevent such acts, but they all have to be weighed against the cost as they affect the rest of our freedoms. Any of these things could be prevented, but we'd wind up living in a society that would terrify Benito Mussolini. Ideally, what we want is to tailor each attempt to mitigate these occurrences to cause the least possible infringement upon liberty.
Of course, that's easier said than done. There are always gray areas, and fuzzy lines like we're seeing here. There isn't any well defined way of handling these things; only best judgement. In these situations, my preference would be to try my damnedest to err on the side of freedom. If there's a problem, address that once it's established. What I'm opposed to is automatically taking the heavy-handed approach, and then trying to take back authority we gave the government later if we decide that it was too much. That never works. I'm looking for incremental approaches to safeguard freedom, rather than the blunt force approach that the law and order types tend to want to start with. One's much easier to tweak than the other.
As all of that pertains to this instance, there's a great deal to be lost by deciding that a person can be forced to divulge information that might convict them. Particularly if you start to apply it to very likely scenarios that will be coming in the future. Much like the recent GPS case and AJ Sotomayer's opinion, we need to be looking forward when establishing what is and isn't acceptable. Personally, I'd rather be safe than sorry.