I would be very surprised if Unity didn't have it up and running in one or two months. They already support ARKit, Windows Holographic, and Vuforia more or less natively. Also, given the ground-level work they've done to enable support for VR without directly dealing with vendor-specific plugins, adding just one more is probably not that big of a deal.
Yeah for the time being, unless you are making a showcase piece or exclusive, then an independent third-party cross platform kit like Vuforia is still the best choice currently due to market size. Vuforia works on phones back to 5s and nearly all androids with cams. Though this is more basic AR based on OpenCV that use target tracking, they do have smart terrain and some VR/AR switching capabilities.
Vuforia's mobile kit for iOS/Android/desktop and has been around since Qualcomm made it starting 2012ish. Though they do not have the point cloud tech and ambient lighting tech as far that make ARKit/ARCore a leap. Vuforia will probably not be able to keep up in the long run but is widely available today because it is hardware independent.
With ARKit/ARCore hardware requirements it will take a few years before there is a large enough market to make a mainstream app not just a showcase AR app/game. For this to truly be something that you can get into games that are cross platform, it will take an independent third party like Unity/Unreal/Vuforia/metaio (before Apple bought them) etc to make it mainstream, or an app architecture that can switch out AR depending on device/capability.
I have launched lots of AR apps, mainly games for kids, using Vuforia, OpenCV, metaio and some other kits, primarily where the AR is an extra feature where people play the game on their desk or unlocks from AR targets/trackers on products. Currently it is a nice gimmick without real-world awareness but great for games.
ARKit/ARCore are better than Vuforia but still locked to a platform which will bring challenges until it is abstracted into a common feature to use in Unity/Unreal/WebGL etc. Exciting progress on AR though between Apple and Google and competition like this always benefits.
There is a lot of discussion here about how Apple has a headstart on developer commitment with ARkit.
What will actually happen for the majority of games targeting AR is people will write it in Unity (or perhaps Unreal), and then set it to compile for iOS and Android.
The S8 was the top selling Android phone this year, so this can be rolled out immediately to phones. I just tested out the sample app on my Pixel. As time goes on, the percentage of Android phones with this capability will increase.
ARkit does not work on iPhone 6 or earlier, or the iPad Air 2 or earlier. It can roll out to a greater percentage of Apple devices right now, but Android has a larger overall market share any how. Two years from now, I expect AR on iOS and Android to be fairly on par (of course we have to see how the two stacks measure up against one another).
