Apple on Tuesday posted an announcement on its iOS Developer page stating that as of February 1, all new app submissions, and even app updates, must be built with the latest version of Xcode 5. What's more, all new apps and app updates must be optimized for iOS 7.
So if you're a developer beholden to the look and feel of iOS 6, it's probably a good time to reluctantly embrace iOS 7 if you want to see your app gain entry into the App Store. I don't imagine, though, that this will be much of an issue given that many developers have enthusiastically adjusted the look and feel of their apps to more closely mirror iOS 7's design aesthetic.
The developer posting was accompanied by a link to Apple's revamped iOS Human Interface Guidelines in case anyone is curious as to what iOS 7 optimization entails.