Private threads?
Either that or just the opposite: public /msg's. Which, you will tell me, amounts to the CB. But the CB is transient too, logs and recordings notwithstanding. And for what is worth, some like to have turned it off, just like me for example.
But seriously, speaking of adjectives like "public" and "private" rings a bell in me reminding me of a grand scheme I have in mind for web sites like forums, wikis and blogs, which IMHO could be incarnations of the same base mechanism founded on an idea similar to that of permissions on *NIX systems - or perhaps of extended attributes. I'm convinced that with a well though and relatively restricted set of such "flags" one could reproduce the semantics of all those environments, and perhaps create useful mixtures between them. Of course like all grand schemes it has the defect of not being implemented that I know of. In particular I was not thinking of such a beast specifically for PM, but in line of principle...