It was only back in February that Apple got in trouble for lax privacy restrictions that lets apps steal your personal data like contacts for example. Now in iOS 6, Apple has improved its privacy features and slammed the door on that sort of activity. Now apps will have to explicitly get user's permission before accessing your personal information.
Here's the exact language from the security section of the iOS 6 release notes:
In iOS 6, the system now protects Calendars, Reminders, Contacts, and Photos as part of Apple's data isolation privacy initiative.
Users will see access dialogs when an app tries to access any of those data types. The user can switch access on and off in Settings > Privacy.
There are APIs available to allow developers to set a "purpose" string that is displayed to users to help them understand why their data is being requested.
There are changes to the EventKit and Address Book frameworks to help developers with this feature.