Younes Manton, the developer that started working on Gallium3D with video support via Google's Summer of Code project, has now moved XvMC under the Xorg state tracker. The X-Video Motion Compensation support isn't far at all, but when complete it will be provided by this state tracker. Younes began work on different Gallium3D video decoding work back in 2008.
While XvMC offloads more of the video decoding work to the GPU during the video playback process compared to X-Video, these days with HD video it hardly is an ideal solution. The real win will come when VDPAU and/or VA-API are fully exposed through Gallium3D. This though is still a ways out.