Haswell commits to the xf86-video-intel X.Org driver aren't as exciting as the Intel DRM driver commits within the Linux kernel or the user-space 3D work for Haswell, but is important nevertheless since there's still more than a half-year until the 2013 Intel processors ship. Aside from work on the Intel SNA architecture, not many interesting things happen these days within the Intel DDX driver since all the important bits are now handled in kernel-space by the Direct Rendering Manager and the DDX drivers are on the way out with Wayland.
Haswell commits against the xf86-video-intel driver can be found via this CGit search. The work committed in the past two hours includes introducing a new chipset identifer for Haswell, noting that for UXA acceleration Haswell hardware can use at least 64 URB entries, shifting around some bits, and other UXA/SNA 2D acceleration changes for Haswell. As mentioned in earlier Phoronix articles, internally the code is recognizing Haswell (a.k.a. "HSW" for short) as Intel "Gen 7.5" graphics rather than "Gen8" (Ivy Bridge is "Gen7"), but this isn't to mean the Haswell improvements are small, as there's really a number of architectural changes with Haswell that will lead to greater graphics performance over the current Intel HD 4000 graphics capabilities.
More Haswell work on xf86-video-intel is needed before the X.Org driver will be ready for next year's Intel processors, but there's still plenty of time left and new xf86-video-intel driver releases come at will (rather than a more defined schedule for the Linux kernel and Mesa).
On the Linux kernel side, the recently released Linux 3.5 kernel appears to have good support for Haswell that should put the DRM/KMS side up to par with Ivy Bridge. By the time Haswell hardware is actually available in the retail space, we'll likely be up to the Linux 3.8 kernel or thereabouts with more changes and optimizations.
When it comes to Mesa, there haven't been any Haswell-specific commits since the end of March when the code was first published, per this query but the OpenGL hardware support should be in good shape for when the hardware ships.
At the same time, Intel Open-Source Technology Center developers continue in parallel with hardware enablement of Valley View for open-source Linux as the next-generation Atom processors begin to ship in the coming months.