Justin Tadlock is a WordPress heavy (wrote the popular Hybrid theme framework), recently released a comprehensive user management plugin called Members, which among other things can turn your blog into a login-only venue:
Private Blog: Allows you to create a
private blog that can only be accessed
by users that are logged in (redirects
them to the login page).
But I get the impression that you're looking to keep some part available to the public users and other parts only for registered users.
You can also use this plugin to restrict certain areas of your blog, screenshot example here: based on role
Hiding the tabs from the public user is the challenging part. Themes typically aren't designed with private portions in mind; you'd have to hack it yourself or get someone to do it for you.