Mir was discussed briefly at this week's X.Org Developers' Conference by a Canonical employee.
Canonical's Chris Halse James Rogers who has traditionally been part of their X.Org maintenance team and recently been involved with their Mir Display Server is in the Portland (Oregon) area and was thus at this week's XDC2013. Chris had a brief topic covering Mir but more specifically their nested X Servers and if there's anything they can share with others -- i.e. Wayland.
There are the PDF slides
and video that's embedded below. Among the only things that were presented for possibly sharing between nested servers of Wayland and Mir would be a generic GLAMOR-based hosted DDX video driver, a window manager proxy, and potentially the XCWM library.
Among Roger's talking points for how Wayland and Mir differ is that Canonical's display server for Ubuntu is just an API rather than protocol, Mir has sever-allocated window color buffers, and it's a focus of "by us, for us" rather than the broad focus of Wayland.