This looks like something Google is trying to do, but in two different products: Google Wave and Google Chrome OS. Pip.io essentially combines these two to make one (imho) revolutionary product.
Thanks Tyler, nice post! Very happy that you're finding Pip.io enjoyable.
So there's hard privacy (only these people can see it), but also a lighter "target" gesture that highlights the post for certain people?
I tend to think of the 'target' as a semi-private, sort of like the @replies with Twitter.