The information has been provided by Mark Shuttleworth in his UDS keynote
Mark Shuttleworth, the founder of Canonical, has started the Ubuntu Developer Summit with a keynote and a very interesting information about Mir, Unity8, and Ubuntu 14.04 LTS.As we all know, Canonical decided not to implement Mir into Ubuntu 13.10 (Saucy Salamander) as the default display server. The technology was too new and unstable and couldn't provide a proper experience.
This decision left the users with the certitude that Mir will definitely be a part of Unity 14.04 (Trusty Tahr), along with Unity8.
Both of these technologies have been implemented in the Phone version of Ubuntu, but Mark Shuttleworth has confirmed, in the keynote, that neither Mir nor Unity8 will be integrated, by default, in the desktop version of Ubuntu 14.04 LTS.
Uses will be able to find them in the official repositories, but for now it seems that Ubuntu 14.10 will be the one likely to carry Mir and Unity8 by default.