Public launch September 17 ahead of iPhone 6 debut.
Apple Issues Invitations for October 4th iPhone Media Event
As noted by The Loop, invitations for the event carry the tagline "Let's talk iPhone" and depict iOS icons for Calendar, Clock, Maps, and Phone.
Apple is of course widely expected to introduce its next-generation hardware (whether it be iPhone 4S, iPhone 5, or both) at the event, and is expected to also be including a minor update to its iPod line. iOS 5 and iCloud are also expected to be topics of discussion at the event, while other reports have indicated that Facebook may launch its iPad app at the event.