It was in October of 2008 when I first extensively talked about X-Video Bitstream Acceleration after some mysterious libraries appeared in their Catalyst Linux driver a month prior. At that point it was possible to use XvMC (X-Video Motion Compensation) with ATI GPUs bearing the UVD2 video decoding engine, but it was not until more than a year later (November of 2009) that XvBA became somewhat official on Linux.
Splitted Desktop Systems released their VA-API to XvBA library that they developed in cooperation with AMD as the XvBA interface is not public. Since that point, it's been possible to utilize the video decoding engine on modern Radeon graphics processors with XvBA, but only by using this VA-API front-end library. The xvba-video package from Splitted Desktop Systems is left binary-only at the demands of AMD, and it works well for some users in conjunctions with Splitted's libva library and VA-API-capable applications like MPlayer.
It's hard to gather what changes may be coming to improve ATI/AMD Linux video playback via the Catalyst Linux driver besides having the driver expose VA-API directly, which may clear up some issues and make it easier to utilize the GPU-assisted video playback. It would be really great if they would implement NVIDIA's VDPAU within their driver, but that's perhaps too optimistic. Unfortunately though this still won't mean anything for those with Radeon hardware on the open-source driver as there is no VA-API / XvBA / VDPAU support over the UVD2 engine. The video decode engine on the Radeon GPUs goes without open-source support or technical documentation as it could lead to compromised DRM (in this case, Digital Rights Management and not the Linux Direct Rendering Manager) on other platforms.