The critical bug-fix found in the driver that was released on Saturday morning deals with page-flipping when the pipe is off, which can lead to some problems in certain conditions.
This driver also re-enables XvMC (X-Video Motion Compensation) support for the Intel X.Org driver although VA-API (Video Acceleration API) is the far superior choice for Intel's open-source Linux graphics driver. XvMC was lost when some defines were reordered that made it not see the XvMC option for the xorg.conf.
Additionally, xf86-video-intel 2.20.9 is already preparing for the X.Org Server 1.14 release that will happen early next year. X.Org Server 1.14 will have more DDX driver API breakage, for which Intel is already bracing.
As usual, there's also a lot of SNA (Sandy Bridge New Acceleration) architecture improvements/fixes.
More details from the Saturday release announcement.