Apple has announced on its developer website that all new app submissions as of February 1, 2014 must be optimized for iOS 7 and built with the latest version of Xcode 5.
The submission standard includes all new apps and app updates.
As a result, there will likely be a surge of app updates before the deadline, as developers race to tweak their apps before Apple starts rejecting apps based on the use of pre-iOS 7 software elements, as well as older versions of Xcode.