There isn't any official release announcement, but in going through the Mir Bzr repo, changes since 0.0.7:
- A connection authorization mechanism has been implemented.
- Continued documentation improvements.
- A change to the Debian packaging so Mir will not build on PowerPC architectures but only x86, x86_64, ARMv7 hardfp, and 64-bit ARMv8.
- Various other changes and fixes.
If you are interested in trying out the latest state of Mir, well mostly XMir, see the Mir Unity Wiki documentation for the procedures until all of the changes have been merged for Ubuntu 13.10.