There's great contribution Admin Profiles, which adds permissions to Admin pages. This one is based on it.

I've changed it to have permissions on categories too. What have been done:
+ Main admin can check the categories and/or any subcategory which is allowed to see by administrator user.
+ When admin has no permission to view category, he don't see it on categories page, can't go there using browser command line & can't find that category and it's subcategories.

You can download it, but it has to be completed.

Changes to many core files has to be done(as kuroi wrote here).

If you have any suggestions or questions, welcome.