Review the iOS 4 Readiness Checklist for a wealth of technical resources to help you make the most of the new technologies in iOS 4 and information on submitting your iOS 4 apps to the App Store.
- Existing apps in the Apps Store and those built using iPhone SDK 3.1.3 should already run on iOS 4 without modification.
- All new applications and updates to existing applications must be built with iPhone SDK 4. Please note, the App Store will no longer support applications that target iOS 2.x.