Starting February 1, new apps and app updates submitted to the App Store must be built with the latest version of Xcode 5 and must be optimized for iOS 7. Learn more about preparing your apps by reviewing the iOS Human Interface Guidelines.
Whether or not that means something as simple as linking to the iOS 7 SDK instead of the old iOS 6 SDK, or whether it means apps have to be redesigned in keeping with iOS 7's depth, deference, and clarity principles isn't exactly clear. However, iOS 7 is meant to be the most tabula rasa version to date, with the chrome getting out of the way so developers can fill the screen with anything and everything imaginable - including green felt if they so wish - my best guess is just that. No more pre-iOS 7 chrome. And bye-bye iOS 6 keyboard...
We've already updated the iMore for iPhone app to iOS 7, and the new frameworks really do enable much, much better software.
Hopefully there'll be some clearer guidance as the deadline approaches. If anyone has any better interpretations, please let me know!