All I know of, is later this year. Very sparse information at every presentation. More of an introduction to what Mesh networking is. Nothing on how integration with the 2600ac router will work.
Unrelatedly, their slide deck (pg 106) does indicate a 5GHz backhaul.
I hope they can manage to integrate Ethernet backhaul capability. Presumably prototypes, but there's seemingly two Ethernet ports and hopefully one can be re-purposed:
I wonder what the price will be; ostensibly cheaper than buying a second RT2600ac and running as a second AP. The RT2600ac's been a steady $200-$220 for over 18 months now.