Canonical's Didier Roche has confirmed that the Wayland display server will be used by default in Ubuntu 17.10. The older Xorg will still be available on all installs if you need to fall back but it'll no longer be the default option. Canonical hopes that this will give people enough time to test Wayland for lingering bugs before the rollout of the long term support release, Ubuntu 18.04, in April 2018.
In his blog post, Roche says:
“Oh, one last thing… we are going to switch over Wayland by default on 17.10. The Xorg session will still be available alongside. This enables us to get some good set of feedback to be ready and make our final decision for our next LTS, 18.04 … All those changes will be rolling out in the next couple of weeks and you can see all changes coming in if you follow artful closely.”
Roche mentioned various changes, referring to what will happen to the global menu, the HUD, alt-tab behaviour, messaging menus, volume notifications, launcher integration via running apps and software centre, lenses and scopes, and other tweaks that don't come with GNOME. Discussions regarding what will will happen to those aspects, whether being baked into GNOME or being made available as extensions, took place at the GUADEC GNOME conference.
Ubuntu follows in the footsteps of Fedora by switching to Wayland. Fedora shipped with Wayland on by default starting with Fedora 25, which launched in November 2016. Ubuntu, the biggest Linux distribution, switching to Wayland, should act as catalyst for other distributions to start switching en masse.