The last time the undeveloped X12 protocol was talked about on Phoronix was back in June of 2010, but to some surprise this morning when waking up there was an update to the X12 Wiki page on X.Org. The Wiki X12 update was by Edward Cullen and it expanded the X12 introduction and rationale while outlining some of the X12 requirements.
X11 was defined at the start of the 1990's. Since then, computers have changed ''almost'' beyond recognition; the simple framebuffer model used by computers at the time has been replaced programmable graphics hardware that is both complex and powerful. Not only has graphics hardware changed, but the basic processing model has and continues to change; parallelism in core system design is becoming the norm, rather than a special case for 'large' systems.
And that's just talking about desktop systems; now there are smart phones, netbooks, tablets and probably will shortly be other device types that this author can't imagine (else I'd be working on them...).
In short, X11 was designed for a different era of computing.
Among the proposed requirements for X12 is security designed from the start, multiple platform support (in terms of hardware platforms from mobile phones and tablets to desktop workstations), maintaining network transparency, support for modern graphics hardware and rendering, maintaining the frame-buffer concept, being as efficient as possible, and thinking on parallel.
There's a lot more information about current X11 issues and other X12 ideas on the full X12 page. So far though there's been no concerted efforts to begin work on X12, nor is it likely we will see such actions in the near future, if ever should Wayland manage to be a smashing success.