For future reference, this is what you have to do now:
Change "Configuring privileges for..." from "Global" to each category. You can then remove all "Guest" privileges.
It's late and I'm probably just not thinking clearly but is it possible to have an entire category listing be hidden to registered users except for a single post?
Alternatively - is it possible to show a block of text above a specific category when a user goes into it? Though I imagine if you have listing disabled they won't be able to get into the forum to even write a post there.. Hmmnn.
Essentially I want a forum that no one but a specific user class has access to list and see what is inside, but any registered user can actually post to that forum (if that makes sense).
Honestly? I'm not sure if this can be done with the privilege system... you could try it?
Give access and topic create permissions to registered users, and nothing else...