Yup. What you actually do is decide on a post by post basis if you want it published or protected. Protected posts are only visible to registered people who are logged in and are also members of the specific blog you posted in.

You can also make your posts private, which means you (or anyone who logs in with your username and password) get to see the post.

Hi,
As a follow on to the above question, I'm wondering if there is a way to keep categories 'private' as well as posts. The suggested solution above will keep posts private but will allow for un-authenticated users to view all categories, albeit potentially empty of posts. Is there a way to edit the _categories.php code such that it only shows categories that have public posts and leaves off those that do not, thereby hiding categories from view if a user is not logged in.
Thanks,