The current open-source UVD support has been limited to the newer Radeon GPUs with the original UVD-bearing graphics processors lacking the support with it having not been a priority for AMD's small open-source team, fears of an open UVD implementation compromising the DRM support for other platforms, and the lengthy technical review process in getting new code published at AMD. Surprisingly, this weekend Christian König published the open-source code for turning on this GPU-based video decoding for the Radeon HD 3000 series.
The patches implement UVD support for the AMD RV600 GPUs, the RS780/RS880, and the RV770/RV790. These patches go against the Radeon DRM kernel driver, which means the code won't be mainlined now until the Linux 3.18 kernel. Additionally, a Mesa/Gallium3D patch is needed along with new UVD firmware/microcode files for the hardware. This support will allow for the original AMD UVD GPUs to expose video acceleration via the Gallium3D VDPAU state tracker, but for this older generation of hardware there is no VDPAU/OpenGL interoperability support. The open-source AMD team is committing to just "best effort" support to this older hardware.
Those wanting to test out the patches can find them on the dri-devel list.