Mir wasn't the exclusive subject of today's Ubuntu X mailing list message, but rather the status and plans for the X.Org Server and Mesa. Bryce Harrington of Canonical wrote X.org 1.14 and RR / 13.04 (un-)plans.
Long story short, Mesa 9.1 should be uploaded to the "Raring" archive in the next week or two. Mesa 9.1 brings improvements and (hopefully) few regressions. Bryce does note that there are some EGL improvements that may benefit Ubuntu Phone and Mir. Overall, Mesa 9.1 is definitely worth upgrading and was released at the end of February.
As announced earlier today, X.Org Server 1.14 is now out in the wild. Usually, when the release of a new xorg-server comes in before the Ubuntu feature freeze (as was again the case this time) and isn't too invasive (1.14 isn't too invasive with minimal new features and mostly fixes / minor improvements), it will be pulled into the next Ubuntu Linux release.
Canonical, however, will not be pulling in X.Org Server 1.14 for Ubuntu 13.04. The basis for this decision is the AMD Catalyst driver not being ready and, of course, Canonical's heavy focus now on phones and tablets. Bryce says, "the general interest of avoiding disrupting folks working on phone/tablet, we're going to hold off on uploading xserver to the archive for a month or two. We may end up skipping 1.14.0 entirely and wait for 1.14.1, but we'll see."
I don't know if we're officially doing a 13.04 Ubuntu release or not, but if we do that means we would stick with the current 1.13.x Xserver series for that, but update to mesa 9.1. It sounds like 1.13 will continue being actively supported upstream, so this seems like a sound option. Since we ship 1.13.x in both quantal and the 12.04 update stack, it would make SRUs slightly simpler as well.As talked about yesterday, the Ubuntu 13.04 release will happen and how Ubuntu rolling releases will happen is still to be firmly decided.
Ubuntu's Mir plans have also already (negatively) affected the amount of time that their developers are spending on X.Org packaging, bug triaging, etc. "We're also starting to balance our time between X and Mir, so if this gives us any extra time then we can contribute help towards Mir development."