On Gitorious is this project for bringing DXVA support over to MPlayer so that H.264/MPEG-4 AVX and VC-1 streams can be done on supportive GPUs and drivers. DXVA is only supported under the Microsoft Windows drivers.
GPU-based Linux video acceleration in MPlayer is already well supported with VA-API, VDPAU, XvMC, and X-Video being among the APIs being supported. VA-API and VDPAU are, of course, the premiere Linux video APIs that are capable of offloading most of the playback process to the graphics processor. Both do a splendid job and cover most of the hardware and drivers, while the Nouveau / Radeon open-source drivers are the only drivers lacking support for either API at the moment, but that will hopefully change soon in the Gallium3D world.
The proprietary Catalyst driver supports interfacing with VA-API applications when using the Splitted Desktop Systems' VA-API to XvBA translation library. X-Video Bitstream Acceleration is the native format AMD exposes by their binary Linux blob for exposing the UVD2/UVD3 engine. XvBA resembles Microsoft's DXVA, so this new MPlayer support may help out anyone interested in a native XvBA implementation for MPlayer, but there's no word of anyone acting on that at the moment.
It would have been fairly interesting if instead there was a VA-API to DXVA library so any VA-API supportive application could support Microsoft's video acceleration format under Windows, but instead, for anyone interested there's now a native DXVA implementation for MPlayer.
Besides Windows itself, DXVA is also used on Microsoft's Xbox 360 game console. DXVA has been around since the days of Windows 2000 but in Windows Vista/7 is DXVA 2.0 that is capable of accelerating more operations, including the video capturing process.