For those not familiar, the quarterly Intel Linux driver package isn't a new software release/component per se but rather the recommended versions of the key software packages making up the Intel Linux driver software stack. These packages have been released in the past days/weeks.
The Intel 2012Q4 graphics "package" recommends the Linux 3.6.5 kernel, Mesa 9.0.1, xf86-video-intel 2.20.12, libdrm 2.4.40, libva 1.1.0, vaapi-driver-intel 1.0.19, and Cairo 1.12.4. Intel's QA team tested this software configuration on X.Org Server 1.13.0, although the xorg-server version isn't officially part of the stack.
Over the recommended package versions from Q3'2012, new features include proper 2D and 3D and hardware-accelerated video decoding support for Intel's next-generation Haswell processors, initial Intel Valley View support, and other improvements throughout. Haswell CPUs will begin surfacing in H1'2013 while the release date for the Valley View Atom SoCs with Ivy Bridge derived graphics aren't yet known.
While the recommended kernel version is 3.6.5, for Intel Haswell support, Intel is recommending the Linux 3.8 kernel. Yes, the Linux 3.8 kernel merge window isn't even open yet let alone the release of the Linux 3.7 kernel, but this is really what's needed for proper Haswell support.
Intel has been working on open-source Haswell driver support for about one year's time in the public Git repositories, but they admitted they screwed up and in terms of good acceleration support and mode-setting it will finally be good in Linux 3.8.
Other features of the 2012Q4 package versions is support for hardware contexts, improved GPU reset handling, improved interrupt support, improved hardware semaphores support, various display fixes, clocking fixes, 2D UXA/SNA acceleration fixes, initial OpenGL 3.1 support, MSAA anti-aliasing, new OpenGL extension support, and much more. The Intel 2012Q4 package should also be in fairly good shape for running Valve's Source Engine / Steam games with Intel HD graphics on Linux.
More details on the 2012Q4 package can be found from IntelLinuxGraphics.org. There's also a few more remarks from Intel's Gordon Jin on the Intel mailing list.