Kevin Gunn, the developer leading the display server team at Canonical, announced this morning that "XMir has landed!" Over the past few days, the various X.Org patches needed for running XMir have been pushed in their respective patches to the Ubuntu main archive. The last piece of the puzzle was landing the unity-system-compositor component, which is now being pushed into universe.
While the X.Org/Mesa patches are now present in Ubuntu 13.10 packages, they still sadly are not mainlined in any of the upstream repositories.
While these components are now living in the mainline repositories and not any separate package repositories, XMir isn't yet enabled by default for all configurations. Before the Ubuntu 13.10 feature freeze they want to round out development by delivering multi-monitor and compisition bypass support. The multi-monitor support is necessary and obvious while composition bypass is needed to avoid double compositing when running fullscreen games on XMir, etc. It's a performance requirement.
Mir for Ubuntu Touch is also still being developed with bug-fixing for window management and Mir-on-Mir support for the Unity 8 greeter/shell.
Beyond this, the remainder of the Ubuntu 13.10 cycle they intend on improving Mir's performance and addressing any visual corruption problems.
More details on these latest (X)Mir advancements can be found on the ubuntu-devel mailing list.