In October, Apple announced that all new app submissions have to support 64-bit ARM starting February 1st 2015. Unity have tackled this issue by investing massive resources getting their IL2CPP back-end working on ARM 64-bit. I can only guess at how exhausting this must have been for the team, kudos to them.

There are a few things that currently don't work, e.g. RakNet, Web Request/Web Client, and any 3rd party plugin that has native code that currently doesn't support ARM 64-bit. Debugging might also be a bit funkier now: you debug C++ in XCode instead of C# in Mono-develop. 

Here's the Announcement with some micro benchmarks that came in yesterday.