Originally the plan was to be using XMir in Ubuntu 13.10 for a Unity 7 experience atop Mir with the X11 compatibility layer, but that was ultimately delayed, pushing back their goal in the next release of using Mir by default with Unity 8. In November it was then revealed that Mir won't be found by default in Ubuntu 14.04 LTS. This all comes four years after they originally were going to use Wayland but then decided to write their own display server. Canonical talked up a Wayland preview in Ubuntu 12.04 and Wayland by default in Ubuntu 12.10, which never materialized.
Mir is still packaged as an experimental option -- along with an experimental Unity 8 desktop session (with very poor results thus far) -- for the upcoming release of Ubuntu 14.04 LTS, but it's not being shipped by default on the desktop. Ubuntu Touch images, however, are continuing to use Mir for their lighter workloads and use-cases.
Canonical has previously committed to Mir in Ubuntu 14.10, but now it looks like that might not even come to reality. During today's keynote by Mark Shuttleworth for the latest virtual Ubuntu Developer Summit, he obviously was talking a lot about Ubuntu's mobile ambitions and then Mir.
Mark said during his vUDS keynote that he expects lots of people will be using Mir as their default display server. He then said that "as soon as it's rock solid" which should be "by 16.04 it will be the default display server." Using Mir, Mark says, will lead to supporting more hardware, obtaining better performance, and "do some great things" with the technology. He expects some users will start using Mir on the desktop over the next year.
Hopefully it will end up being enabled by default in 2015 to allow for widespread testing and vetting ahead of the 16.04 Long-Term Support release. From today's keynote it sounds more and more likely that Mir will again be delayed from shipping by default in Ubuntu 14.10 for October -- well, extremely unlikely at this point. This obviously will give more time for upstream driver support too, which for now is non-existent. The binary AMD and NVIDIA Linux graphics drivers don't yet support Mir but not even the upstream open-source drivers have the support... Mesa hasn't yet pulled the Mir back-end support and there's outstanding patches to support XMir by the X.Org DDX drivers, which so far have been rejected. The Ubuntu graphics driver packages as a result have had to be carrying their own out-of-tree patches for compatibility. Additionally, no attempts have yet been made for mainlining XMir within the xorg-server.
Those wishing to watch Mark's vUDS keynote can watch it below.