In total there are more than 100 changes in xf86-video-intel 2.6 RC1 over the xf86-video-intel 2.5 series. While X-Video Motion Compensation (XvMC) support is present in this driver release up through the X4500HD IGP, it's currently disabled by default when using the Direct Rendering Infrastructure 2.
DRI2 and RandR 1.3 support can be found in X Server 1.6, which is scheduled for release in January. Support for the Graphics Execution Manager is being introduced with the Linux 2.6.28 kernel while support for kernel mode-setting is being brought with the Linux 2.6.29 kernel next quarter. The release of xf86-video-intel 2.6.0 is also now expected in Q1'09.
The release announcement and source download link for xf86-video-intel 2.6 RC1 (v18.104.22.168) can be found on the X.Org mailing list.