In particular, as it *might* even be the case that Valve is actually the driving force behind this.
The whole timeline looks a bit suspicious to me.
They started the first internal commits in mid 2012.
Negotiations with Canonical about the Linux base system of the SteamBox could have happened in early 2012.
Also, they aim for a first (productive?) release end of 2013.
That agrees with the planned release of the SteamBox, end of 2013/early 2014.
And how the hell did they manage to convince AMD and Nvidia (especially AMD) to join if it wasn't for the Steambox?
Seriously, Ubuntu alone wouldn't make them support Mir and probably cut Wayland support.
I can't think of AMD supporting Mir and Wayland. That may be different for NVidia, though.