The Xubuntu team hasn't decided yet if the Xfce Ubuntu flavor will switch to XMir or continue with X.org for 13.10. Before making the decision, this needs to be properly tested so an Xubuntu ISO running on Mir & XMir has been made available for download.
According to a message posted on the Xubuntu-devel mailing list, the team will decide if Xubuntu 13.10 Saucy Salamander will switch to XMir later this month:
It is of EXTREME importance that we do as many tests as possible as the technology evolves. We need to understand how our system will behave under XMir in order to make the decision later this month whether to move our standard ISO from X.Org to XMir.
XMir is an implementation of X running or Mir, the display server developed by Canonical. As you probably know, Ubuntu 13.10 Saucy Salamander (the main flavor with Unity) will use XMir by default, while Kubuntu and Lubuntu will not switch to XMir for 13.10. There's no news on Ubuntu GNOME and XMir for now.
If you want to help test Xubuntu on XMir, grab the ISO from HERE (you don't need to install it, simply create a bootable flash drive or write it on a DVD).