Windows Update has begun hosting Windows on ARM files (via MSPU). Microsoft briefly mentioned Windows 10 on ARM at IFA 2017, with a possible release date in late 2017, possibly early 2018. The MSPU story notes that there were some files supporting Windows 10 ARM64 build 16281 were spotted by one of their tipsters on Windows Update servers.
Interestingly enough, the ARM64 files have different SKUs that are separated by different Windows 10 operating systems, including separate files for Windows 10 S, Enterprise, Pro, and others. Unfortunately, these builds are not individual ISO files, so you can't just download and install the ISO files. Rather, there are files that are a part of Microsoft's Windows 10 Unified Update Platform (UUP), so you would have to have all the different UUP files together to be able to make the full ISO file for each individual Windows 10 OS.
Since the files were spotted on the Windows Update server, they might be able to be used as part of the Windows Insider program, just like on Windows Phones and Windows 10 PCs. Of course, Microsoft has already said that current Windows Phones will not be getting Windows 10 on ARM.
Per the MSPU story, here's a list of the ARM64 file names:
As we already know, the Windows 10 Fall Creators Update is coming October 17, 2017. It is possible that Microsoft is planning to release Windows 10 on ARM around the same time. Panos Panay will be delivering the keynote at Future Decoded in London in October and probably announcing a new Surface device too. Hopefully, Windows 10 on ARM devices won't be far behind.