Review the iOS 4 Readiness Checklist
The next generation of the world’s most advanced mobile operating system is in the hands of iPhone users around the world, and is now available to iPod touch users for free. Make sure that your applications are compatible with iOS 4. All new applications and updates to existing applications must be built with iPhone SDK 4. In addition, the App Store will no longer support applications that target iOS 2.x. 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.