Canonical has announced, through the voice of Oliver Ries, that Mir and Unity8 will be default in Ubuntu 14.10.
The plans to integrate Xmir in Ubuntu 13.10 and 14.04 had to be abandoned by Canonical a while ago when the developers realized that it's not ready to be integrated without any problems. The Director in charge of Mir, Unity, and other “things”, Oliver Ries, has explained the plans regarding these components.
“Going forward we are expecting to see Mir being part of the default display stack in 14.10 and following releases, where the key focus is to bring the vision of convergence to our users."
"The received feedback puts us in a strong position to align with our partners on our shared goal of providing the next generation of user experience on top of Mir.” said Oliver Ries on the mailing list.
The same goes for Unity8. Ubuntu users will have to wait a little more to get that new Unity look on their desktops, but I think it will be worth the wait.