Yesterday, Google posted the latest factory image for the Nexus 9 (the third one so far), but still had yet to release a factory image for the Nexus 6. Well, LRX21O just went live on the Google Developers site, marking the first available image for Shamu.
This is the version Nexus 6's purchased from retailers today are being upgraded to out of the box, as well, many of which shipped with the LNX07M build, presumably a considerably older ROM.
You can download the driver binaries here, too.
The Nexus Player also has an updated image available, LRX21V - you can get it here.