Part of the beauty of open-source software is that anyone is free to work on whatever code they wish. For some developers this means working on open-source graphics drivers for old GPUs, for some it may be working on a game or desktop application, and for others it may mean bringing Microsoft DXVA support to open-source multi-media software. There’s now a branch of MPlayer that supports Microsoft’s DirectX Video Acceleration API.
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.