At a recent industry conference in San Francisco, a panel presented the current state of ARC implementation and deployment. A version of that presentation is now available for review – you can download it here. As reported yesterday source code and a test suite are available now, AOL and GMail are validating ARC headers in messages they receive, and commercial products are starting to incorporate ARC – in fact, MailerQ from Copernica has already included ARC for a few months.
If you are looking for more technical detail on how the ARC protocol works, view this presentation from October 2016. Or read the current version of the protocol published by the IETF DMARC Working Group.