Hello Linux Geeksters. As you already know, there are different opinions on Mir and XMir, the open-source display server developed by Canonical. While Ubuntu decided to switch to Mir and XMir, Kubuntu and Lubuntu declined this change.
In short, Canonical switches to Mir and automatically replaces the X.org server with XMir, an X server running on Mir, on Ubuntu 13.10 Saucy Salamaner. Because there are some graphics driver developers that did not agree to include Mir support for their products, an X.org server fallback version will be available for Ubuntu Saucy, in order to get some graphical cards to work.
Jono Bacon, the Ubuntu Community Manager created a video, to demostrate the XMir on Mir usability on Unity 7, GNOME 3, KDE, XFCE and LXDE.
XMir on Mir is not included yet in the daily builds of Ubuntu 13.10 Saucy Salamander, but I think it will be added any moment from now.