Hello,
just found out at http://code.google.com/soc/2008/xorg/appinfo.html?csaid=ACD6AA025594454A that : "he purpose of this project is to produce a video decoding solution for GPUs that are supported by the Gallium3D driver framework. The project will attempt to implement the XvMC API using the programmable pipeline of a typical GPU, thereby providing accelerated video decoding to a wide variety of hardware. Since the decoding will be implemented using the GPU's programmable pipeline, it is important to note that this solution should support all recent GPUs regardless of whether or not they include dedicated video decoding hardware. It is hoped that this GPU-based acceleration will allow for real-time play back of HD video streams on even modest hardware. The implementation will be developed and tested using Gallium3D's SoftPipe driver, a stable software reference implementation, and later Nvidia hardware and the Nouveau driver."
You can follow the progress at http://www.bitblit.org/gsoc/gallium3d_xvmc.shtml
Could be pretty usefull to lots of us :-)