Last year the talked about reason that the umbrella X.Org Foundation umbrella project wasn't accepted for Google Summer of Code came down to the "ideas page" for possible projects not being too interesting to the Google evaluators. There was also talk of the application not being submitted on time and some other possible issues, but the lack of Google not accepting this very important project to the Linux desktop was because of the ideas page.
There was talk again this year of the X.Org Foundation applying once again and there was some progress to the "Summer of Code Ideas" Wiki page. Listed 2013 ideas were Glamor 2D acceleration performance tuning, Glamor X-Video support, work on the Mesa GLSL compiler, R600g OpenCL / LLVM improvements, Clover OpenCL work, GLSL 1.30 for older chipsets than Intel Sandy Bridge, Piglit improvements, Shatter (Xinerama replacement) for the X Server, custom gesture recognition for X Input, and working RenderNode support.
There's several interesting projects on the 2013 ideas list, but when checking out the accepted organizations page, the X.Org Foundation isn't listed. Assuming the X.Org Foundation submitted their GSoC organization application on time (no idea if that's the case or not, since the X.Org Wiki has been shambles for week along with the organization itself, so X.Org Foundation meeting minutes are unavailable right now...), it's sad to see Google deny their entrance once again.
On a positive note, at least the X.Org Foundation does have their own "Endless Vacation of Code" project whereby it's similar to GSoC and they do financially compensate students, but EVoC is run whenever a student applies for a project and is much more loosely organized. Sadly, EVoC is barely promoted at all by the X.Org Foundation... There's only ever been a couple students applying for X.Org EVoC during its several years in existence. For any programming students wishing to learn more, visit the XorgEVoC Wiki page.
Some of the interesting open-source organizations accepted to this year's GSoC program were XBMC, WINE, NetBSD, FreeBSD, ScummVM, QEMU, Mono, KDE, GNOME, and Coreboot, among many others.
UPDATE: The X.Org Foundation is apparently accepted to GSoC 2013 but the organization isn't yet being listed on the Google Melange web-site.
UPDATE 2: The X.Org Foundation is indeed now listed on Google Melange as a participating organization for GSoC 2013.