While recently there was the merging of Mesa pipe-video to master. most of the recent video decoding work going on within the Gallium3D world has been into the ATI/AMD R600 Gallium3D driver for XvMC and now VDPAU support too. This is after the R300 support matured a fair amount, but the first one to the Gallium3D video decoding party was Nouveau. Worked on several years ago as part of the Google Summer of Code was Nouveau Gallium3D video coding. Fortunately, some of this work has been resurrected.
Maarten Lankhorst wrote to the Mesa mailing list
that he has managed to get video acceleration working on his NV96 GPU. He attached his patch, but he's still working on various improvements and settling for the Nouveau interface. He also pushed his code into his own Git repository for those using the Nouveau driver that wish to try out this XvMC support. At this time, the work is only for XvMC and not VDPAU or VA-API.