Posted to the Ubuntu GNOME Google+ page was a Q&A about how Canonical pushing Mir in Ubuntu is irrelevant to the Ubuntu GNOME derivative for now. The post said Ubuntu 14.04 is using X.Org, it's not going to disappear over night, and "Ubuntu GNOME 14.10 will likely continue using X, even if Ubuntu switch to MIR."
The post concluded with, "At some point in the distant future we will probably switch to Wayland, but right now its not even close to being ready." This comes as GNOME support continues to improve on Wayland. GNOME 3.10 on Wayland was decent for starters but with GNOME 3.12 next year the desktop stack should be in much better shape. Wayland 1.4 is also due next month with new features.