I've heard about that case, and I'm a bit ambivalent about it. It's the "yelling fire in a crowded theater" argument (which I think was Oliver Wendell Holmes or Louis Brandeis, can't remember which).

I don't think we should regulate speech based on someone's reaction to it. I have no way of knowing what scares someone. And, would the same comments be illegal if the recipient laughed them off as hoax? You get situation where there is the same speech, different reaction, different legal standing.

As soon as the freak who is doing the threatening does something illegal, then throw the book at him. But speech should not be illegalized, regardless of the content.