Well, I installed RC3 today and copied the basic settings from my RC2 test site, and immediately had the same problem.
Here is the link to the RC3 test site:
http://www.navyjrotc.us/ia/portal.phpAs an unregistered user, you can go to the forums and see the "Development" category, and the forum within it called "Development ideas & discussion". You will notice that you have permissions as an unregistered visitor to post within that forum.
That forum is set up as a Moderator [Hidden] forum. Here is an image of what the set-up looks like:
[align=center:plia32kx][flash=,:plia32kx]http://www.navyjrotc.us/ia/files/iaforum.jpg[/flash:plia32kx][/align:plia32kx]
Can you think of any reason that this forum would be visible to unregistered users?