Stéphane Marchesin, the founder of the Nouveau driver project that went on to work for Google on their Chrome/Chromium OS team, has served as the liaison between X.Org and Google for the annual GSoC. This year, even with Stéphane's continued involvement as a Google employee, they have not accepted X.Org. He shared the bad news on Friday afternoon via the X.Org mailing list. X.Org's involvement in the past with GSoC has included Mesa and Wayland projects as well, just not the X11/X.Org Server.
It seems that Google has reduced the number of participating organizations this year, but for whatever reason has decided to reject X.Org Foundation, a critical organization to the success of the Linux desktop. The accepted organizations can be found on Google Melange. Among the well known projects that were accepted include the Apache Software Foundation, Blender, Debian, FreeBSD, Gentoo, GNOME, KDE, Mozilla, Pidgin, and even Twitter and Haiku.
There does seem to be a number of far less well known open-source organizations, but some at least seem to have a hand with Android. Among those organizations are Apertium, Benetech, Catroid, e-cidadania, Evergreen, and OpenMRS.
At least the X.Org Foundation does have the Endless Vacation of Code. The X.Org EVoC is inspired by Google's Summer of Code, but is organized by the X.Org Foundation, has no time constraints, and is more up to whatever the student wants to do for around $6000 USD. However, because it's organized by the X.Org Foundation and not Google, it's far less organized and barely at all advertised and thus not nearly as popular.
As far as I am aware, in the years that X.org EVoC has been around, there's just been one student to come forward to take up the foundation on their offer. This lone project as the Gallium3D OpenCL work that began last year and ended earlier this year with some success.
With X.Org not being accepted by GSoC 2012, hopefully the foundation will better promote its Endless Vacation of Code and that there will be some students to take them up on their offer. Just on Friday was talk of a Russian student working on multi-GPU and remote display support for Wayland.