I heard from MT staff they are planning on implementing this, but is on the back burner unless they feel a lot of demand for it. I'd love to be able to mark certain entries as private, which would not show up on the blog and not in the feed unless the user was logged in, and their account marked with access to private entries. I can't seem to get the private plug-in to work, and it seems just about everything is already set up in MT to allow this. Just need a field added to the entry page, and a new user level for private entry read access.