This week is the feature-freeze for Ubuntu 13.10 so the Mir developers are trying to squeeze in as much last minute work as possible. There's been more than 50 commits since the Mir 0.0.9 tag (but at the same time there's been many 0.0.9x tags for Ubuntu packages), so check out the Bzr repository if you want to learn more.
Separately, the Mir developers are calling for multi-monitor testing to be carried out using the Mir packages from their System-Compositor-Testing PPA. There's still more multi-monitor work to land for the Mir display server so up until the feature-freeze there's a request for testing from those packages.
Those wanting to engage in this testing can find out more information from this Mir Wiki page. Even with the latest code, features currently not working are resolution changing for built-in displays and secondary displays and also rotation changes for the display orientation, but aside from that they're mostly interested in dual-monitor testing namely with laptop configurations where you'd be connecting a secondary display within an office desk or to a projector in a conference room.