In the last weeks, Canonical has announced that Ubuntu 13.10 Saucy Salamander will ditch the X.org server and use it Mir and XMir instead, a display server developed by the Ubuntu team.
As you already know, LightDM is the default display manager on the latest Ubuntu versions. LightDM has been just updated to LightDM 1.7.4 and got support for Mir-based Unity.
I think that LightDM will bring Mir support to Unity 7, because Unity 8 is about to be used only on Ubuntu 14.04, or at least this is what we know, at the moment.
To install / update to LightDM 1.7.4 on Ubuntu 13.10 Saucy Salamander, just do:
$ sudo apt-get update
$ sudo apt-get install lightdm
Mir has divided the Ubuntu developers into different teams, Kubuntu switches to Wayland and refuses Mir, Lubuntu keeps X.org, Ubuntu adopts Mir, we don’t know what will Xubuntu do, but I have a hunch that it will also keep X.org, since it brings better performance than Mir (at least for now).