Mir 0.3 was released just a few weeks ago while Canonical developers are already out with the latest release. Mir 0.4.0 brings several new features including a surface attribute for visibility, a surface orientation API, and a number of changes to the Mir Server code. Both the Mir client and server ABIs were bumped by v0.4.0. More details on the 0.4 release can be found via Mir on Launchpad.
Mir 0.5 is meanwhile now under development with the latest Bzr code. Landing so far for Mir 0.5 has just been some small changes and fixes while more is expected in the coming weeks.
What are you hoping to see out of Mir in Ubuntu 14.10? Let us know in the forums. Of course, Mir isn't going to replace the X.Org Server on the Ubuntu 14.10 desktop by default but will just primarily target Ubuntu Touch/Phone and be an experimental/developer option in the main archive for the desktop. Canonical is still hoping for first-class driver support and for it to be the default on their Linux desktop sometime by early 2016.