The initial release of unity8-desktop-session for Ubuntu 14.04 has been pushed into the trusty-proposed archive as of today. This package provides Unity 8 desktop sessions for both Mir and X11 and is the initial Ubuntu release to appear in this next Ubuntu version. We'll be trying out Unity 8 on the desktop and likely be delivering some new benchmarks atop Mir in the days ahead.
Unity 8 with Mir will not land by default on the Ubuntu Linux desktop until at least Ubuntu 14.10 with 14.04 LTS still using Unity 7 and an X.Org Server by default. Our most recent XMir benchmarks on Ubuntu 14.04 were from last month in this article while more tests will be done shortly from this exciting Ubuntu update.