iOS 6 brings a lot of new features with it, including granular Privacy controls in Settings. Given previous controversies surrounding apps accessing your contacts, it's important to make sure that only apps you specifically allow have access to your contacts, and only for as long as you allow it. With Privacy, you can grant and revoke permission to access your contacts at any time.
If an app is requesting to access your contacts, it'll normally present you with a pop-up. Some apps legitimately require access to your contacts to function properly. For example, Skype needs to access your contacts to place calls to the people on your contact list. If you deny access, you may find a time later on that you need to grant it. Y
If you accidentally granted an app access to your contacts or decide later on that you don't want that app to have access, you can easily disable it as well.
That's all there is to it. You now have very good, very granular control over your contacts and the private information contained therein.