You can submit 64-bit apps for iOS 7 today that take advantage of the power of iPhone 5s. Xcode can build your app with both 32-bit and 64-bit binaries included so it works across all devices running iOS 7. If you wish to continue to support iOS 6 then you will need to build for 32-bit only. Next month we will be making changes that will allow you to create a single app binary that supports 32-bit on iOS 6, as well as 32-bit and 64-bit on iOS 7.
As Apple states, developers who wish to support any iOS below version 7 must keep their apps 32-bit. However, the company will allow developers to release one build, supporting 32-bit chips on iOS 6 and 64-bit chips on iOS 7, starting in October.
The 64-bit chip in the iPhone 5s is the first 64-bit chip in a mobile device and Apple says it is twice as fast as the 32-bit A6 chip found in the iPhone 5 -- and a whopping 40 times faster than the chip found in the original iPhone.