Quoting Joachim Durchholz, from the post of Wed, 30 Jun:
Finally, there's demand for "strictly internal" scratch pads, i.e. pages that aren't even readable unless you're logged in as a member of a specific group.
as Brion mentioned, MediaWiki is built for openness, and will require quite some tweaking. last time I looked at Twiki it seemed like a much better platform for such permission schemes, give it a look.