Thanks Milan,
There is another possible solution:
Unregistred users see category as a blue link
which will lead them to the list of the articles in the category
but registered users will see red link which leads them to the 'edit
category' editbox
This makes sense _if_ the wiki is set up to allow edits by registered users
only - however many aren't setup this way (I'm not sure statistically which
is higher).
Personally, I think that in general a wiki should be have the same for
registered users and non-registered guests except where it's absolutely
necessary to draw the line (for example if edits are restricted to users).
But that's just my two cents.
-- Jim