Linux and hardware acceleration...
Well, it exists, actually, with a free implementation - now, what one requires is driver support. Heh.
Right now, video acceleration is done through the X video (Xv) extention. Most current drivers support basic Xv: video scaling, YUV conversion and output (overlay, blit, or pixmap). Xv was originally designed to get and send video fluxes, but this functionality is now rarely used.
OpenChrome (for VIA Chrome hardware) has a very complete and Free implementation of X video Motion Compensation (XvMC): inverse discrete cosine transform, and motion compensation - typically, these help in MPEG-2, MPEG-4 ASP and AVC decompression (and since VC-1 is a primitive form of MPEG-4, it can be accelerated too, but for now, the open decoder is too basic to use it).
Another extension is being considered: due to its age, XvMC was never designed for more complete video acceleration than what is required by MPEG-2; as such, Intel is leading the development of a Free specification for video acceleration (called libva, hosted at freedesktop.org), regularly updated).
The OpenChrome's implementation of XvMC is being considered for porting to Intel, Matrox and AMD hardware; as there is no feature complete Nvidia Free driver, XvMC won't be supported on Nvidia hardware for a while (the Nouveau reverse engineered driver still doesn't have enough 3D features to support XvMC properly).
One thing to note though, is that under Linux+X11 Xorg, Free drivers usually share code - as such, a feature's quality is identical across the range.