Unity now added iOS 64-bit support as part of its latest update just in time for Apple's February 1st deadline.
From the start of next month, all new new App Store apps must have 64-bit support and be built with the iOS 8 SDK.
Unity support has been achieved using its new IL2CPP scripting tech, which will enable developers to make a universal build that works on both 32-bit and 64-bit iOS devices.
Issues still remain however and Unity has admitted that some of these may cause delays for some developers. For example, enabling the Xcode Internal profiler will result in compilation errors when using the IL2CPP scripting backend, while RakNet networking has not yet been ported to IL2CPP.
Developers with apps already on the App Store have been advised to continue shipping updates using the Mono backend on ARMv7, which Apple has said it will allow until June 1st. Unity added however that it also recommends devs begin testing their project using the IL2CPP backend as soon as possible.
You can keep up to date with the latest Unity patches here.