We’ll release the Metal code in GitHub as soon as the Apple code NDA’s are lifted, and it will subsequently be available through the launcher in the next UE4 release to follow. We are guessing the GitHub release will coincide with the public release of iOS 8, and we’ll let everyone know if there is an opportunity to release it earlier.
We’re really excited about the things Metal makes possible as a result of its 10x increase in the CPU efficiency of rendering. It really blurs the line between what’s possible on high-end computers and iPhone/iPad!