See Gallium3D Pipe-Video To Be Merged To Mesa Master for the Phoronix explanation what the pipe-video branch is all about, but it basically comes down to bringing video playback acceleration into the Gallium3D GPU driver world. There's already XvMC working on the AMD R600 Gallium3D driver, early work on a VDPAU support, and much more.
The merge happened this afternoon in this commit where it's now living in master.
Merging this branch affects the Xorg, VA, and VDPAU state trackers. VA targets are added for the R300 and R600 targets along with VDPAU and XVMC targets for these two ATI/AMD Radeon drivers too. Bringing in pipe-video adds around 19,000 lines of code to Mesa. The Radeon video acceleration is done using shaders and not the UVD/UVD2 (Unified Video Decoder) engines found on modern Radeon hardware, since that has yet to be publicly documented.
Prost to König and others involved with this large video undertaking for Gallium3D!