Possible Duplicate:
Any way to send a personal message to another user?
The stackoverflow conversations are oriented around questions, but I think some conversations could be more productive on a personal basis with the question context removed.
The one example that I did come up with is that you would like to interact with a user directly. You do not want to misuse a question as your discussion board.
I think this channel should be public. Back room discussion have more disadvantages than advantages.