Same question from me! I would really appreciate knowing this one.
I was able to get this partially accomplished using the Members Only plug in. Also, it helped a lot to put all my forums in a category, even though I do not have very many forums. At least, now the only thing my non-users will see is the goofy message “Oh brother. No forums where found here”, even though I do indeed have forums.
HOWEVER, I still have the problem of the Forum Menu item appearing in my menu bar. I don’t know how to keep it out until users log in.
I’ve spent the better part of a day fiddling with it and still it is here.
If I can figure out which file has the “Oh Brother!” text in it, I could at least change it to say something like “This area requires user authorization”.