Canonical has announced development of a new open source display server called Mir. This server is intended as "a replacement for the X Window server system to unlock next-generation user experiences for devices ranging from Linux desktop to mobile devices powered by Ubuntu."
According to the Mir spec page, the purpose of Mir is to enable the development of the next generation Unity and, contrary to some previous speculation, it will not be based upon the Wayland display server and protocol, which has received attention recently as a possible X replacement.
The Mir specification page gives three principal reasons why Canonical wants to replace the venerable X Window, which has served the Linux and Unix communities for 30 years:

• X Window shares too much system state across process boundaries.
• The complexity of X Window leaves room for applications to spoof input events they don't own.
• The compositor hierarchy ends on the session level. (The graphical shell starts after boot, with no ingrataion of the shell from boot time).

Canonical's grand vision of a single platform for mobile and desktop systems has already led to the development of the homegrown Unity desktop; Mir now takes that  integration down deeper into the system.
The roadmap states that, by May 2013, the project intends to integrate "Unity Next with Mir and provide enough facility to start iterating the actual shell development, providing developers with a solid platform and designers with means for rapid prototyping." By October 2013, Unity Next and Mir window management should be completely integrated to support an Ubuntu Phone product. According to the spec, the developers want to fully replace X in user sessions and provide a legacy mode to support X clients. By April 2014, they plan to achieve "complete convergence across the form factors."

The Mir project will produce the libmir-server and libmir-client libraries.