As a reminder, Ubuntu 13.10 Saucy Salamander will be released on the 17th of October, 2013. The initial plan was to released it with Mir as default, a display server developed by Canonical, used as a replacement for X.Org.
But Mir/XMir is not stable enough for Ubuntu 13.10, so Canonical has postponed the usage of Mir/XMir as default. The problem is with XMir, it does not have support for many applications that work well on the X.org server and has also multi-monitor issues.
Mir will be released only on Ubuntu Touch 1.0, which is scheduled for the 17th of October, the same day as the Ubutu Saucy release.
However, the bleeding edge technology enthusiasts can still test Mir easily, because it is available via PPA. How to install and test Mir 0.0.12 on Ubuntu 13.10 Saucy Salamnder:
$ sudo add-apt-repository ppa:mir-team/staging
$ sudo apt-get update
$ sudo apt-get install mir unity-system-compositor
We don’t know yet if Ubuntu 14.04 will be using Mir as default, but Unity 8 should bring Mir-native support, while Unity 7 doesn’t.