Interesting... I wonder what the applications of this would be for those of us in apartments with paper thin walls? Let me crank up the music, but no one else can hear it? Sounds fantastic.
Just my opinion, but I don't think this idea is going anywhere, nor should it. Even if it did work, just think of the possible abuses of the system (e.g., someone maliciously focuses a super-loud sound beam at someone else and they either crash their car or their hearing is permanently damaged...).
Now we're going to wonder if some of those insane people really did hear "voices" or are thinking they heard "voices" that told them to do stupid things.
From a halloween toy application this would be cool, you could make a "Chuckie" doll that can only be heard if you're directly in front of it and can pull some neat tricks on people ("can't you hear hear that doll screaming?!!".