Daniel Vetter of Intel's Open-Source Technology Center sent in a drm-intel-fixes pull request that provides bug-fixes for the Intel DRM kernel driver. The main change as part of this pull, which is also for current Linux kernel stable series, is the disabling of Haswell (and Valley View) support by default and hiding it behind the previously talked about kernel module parameter.
Vetter's interesting statement though was: "We've aimed for basic hsw support in 3.6, but due to a few bad happenstances we've screwed up and only 3.8 will have better modeset support than vesa. To avoid yet another round of fallout from such a gaffle on for the next platform we've added a module option to disable early hw support by default. That should also give us more flexibility in bring-up."
Intel admits that it botched the early Haswell Linux support, which has been worked on publicly since March although the actual Haswell hardware won't begin shipping until H1'2013. Intel has had the support now within their DDX driver, Mesa, and Intel DRM driver, but the DRM driver is still being problematic.
The big issue it seems for Haswell with the Linux Direct Rendering Manager driver comes down to shoddy mode-setting right now and there isn't working Embedded DisplayPort (eDP) support in the current kernels. So basically your display might not even light-up if you happen to be toying with some Haswell engineering samples or for some reason are using old kernels when Haswell is introduced next year.
With the Linux 3.8 kernel is where everything will hopefully be ironed out. For Ubuntu users, this means there won't be good "out of the box" support until Ubuntu 13.04, which might be a problem depending upon whether Haswell processors surface before or after next April. For the Valley View Atom SoCs this will likely be a similar story with needing Linux 3.8+ for good support.
Let's hope the timing of the Ivy Bridge successor for once isn't too soon or many Linux desktop users may face a poor out-of-the-box Haswell experience. Let's also hope all of the Haswell bits are in fact worked out for Linux 3.8 and there aren't anymore last minute changes required.