i was thinking that adding a filter to the get_archives function would be very useful.
i use a plug-in called post-levels, which lets me assign levels to users and posts. any user with a level equal or greater than the post's level can view that post. i use it to hide posts from non-registered or non-admin users, etc.
the way it works is that it marks a post as "private" and then adds a meta value containing the post's level
anyway, i've fixed post-levels to added a category filter, so that only categories appear, if the user has permission to see at least one of the posts in that category.
i hacked template-functions-general.php to allow a filter for categories, but thought it would be nice if you added it to a future release and gave it an "official" filter name.
thanx in advance.