https://launchpad.net/~mir-team/+archive/staging and click on Technical Details about this PPA and a panel will open and you will see Choose your Ubuntu Version. We have a choice of two: Raring (13.04) and Quantal (12.10). At the moment it might be more vapourware than software but the existence of a PPA indicates that code is going to come down the turnpike. There are couple of Ubuntu wiki pages as well regarding this stuff. That tells me that MIR and Unity Next are not skunk-works ' projects.
In the past something like this would have been held back until the next development cycle or the one after that. Could the releasing of this information and the setting up of a PPA be part of Rolling development of Raring on and on?