Apple issued new developer guidelines on Tuesday. Starting February 1st, all new App Store submissions, including new apps and updates, must be built for iOS 7 with the latest version of Xcode 5.
From Apple's Developer Connection:
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.
MacRumors noted that "optimized for iOS 7" doesn't necessarily mean "look like iOS 7." There are a number of new technologies in iOS 7, and Apple's new requirements mean that new apps and app updates must simply be built to be compliant with iOS 7's infrastucture.
Apple has historically pushed its developers forward with new OSes, and it's one of many reasons Apple's user base updates to the newest OS at levels other desktop and mobile platforms can only dream about.