Gunning For Safety is reporting that Apple is working on integrating Facebook into the iPhone OS 4.0 release. A picture of a system file on the iPhone has surfaced, part of the Settings app on the iPhone. This file describes adding accounts for mail, contacts, calendars, notes, and, etc.
Right now, iPhones have user accounts for Gmail, MobileMe, Yahoo!, and Microsoft Exchange. It looks like Apple is going to add Facebook to that list.
It also looks like Apple wants to implement Facebook‚Äôs Events into the iPhone‚Äôs Calendar app, as well as add Facebook‚Äôs friends network to the iPhones contacts database. Apple also looks to have created a separate type of contact for Facebook contacts.