Your plugin works great, easy to implement, easy to use.
One question, though :
Suppose I am a user and I want to post 'privately' to the admin. So, I am just checking the privacy box, send my post and then only the admin (and myself) will be able to read it on the bbpress forum (the post background will turn blue).
Suppose now that I am the admin and I want to REPLY TO THIS USER regarding the private post that he just sent me. Then, I am checkin the privacy box down my answer and send it on to the forum.
Unfortunately, my answer will NOT be visible by that PARTICULAR user, it will be marked as 'blue' and 'private' to ALL users (but myself) !
The result is that I will be the only one to be able to read my answer !
Am I missing something here ?