While there apparently was no large announcement and it looks like they've been a member dating back to earlier this year, Canonical is a member of the Khronos Group and they can be found on the contributor member page.
Canonical's specific involvement with the Khronos Group isn't listed and we haven't seen Canonical names closely associated with any major specs out of the different working groups to date. However, Oliver Ries, the Head of Engineering Product Strategy at Canonical, wrote into Phoronix that they joined the group for pushing their display server agenda with trying to work towards an underlying driver standard for Mir/Wayland. Oli noted in his email, "Canonical has joined Khronos in order to help establish the necessary driver standard that is required for Mir (and Wayland) to succeed. We have specifically contributed to the current standard proposal/draft."
It's great to see Canonical getting involved with Khronos and hopefully we'll see more out of them with Khronos' OpenGL-Next and ideally increasing their contributions to Mesa/Linux-graphics driver development.