From what I can see i this thread there are a few things to note:
- XBOX is well dead or close to it in regards to video playback capabilities due to it's lack of 1080p playback.
- Apple TV is a cute spin-off but not really the main focus though for now it is worth keeping it alive.
- Linux has some GPU acceleration but this is just for Nvidia cards AMD and others are out in the cold.
- Windows XP, Vista and 7 all have somewhat different ways of working with acceleration of video using DXVA but all of them should have the basic DirectX9.0 options though not ideal there might be some posibilities to get this working.
The main problem with the Windows way of working is the fact that one would need a special build that works just on Windows but would add (some/a lot of) extra code to XBMC that would be useless on the other platforms. On the other hand the Linux acceperation has been added and that seems not to cause to many issues with respect to source bloat for the other platforms.
I would thus suggest to elliminate this as a reason not to bother with DXVA.
The remaining problems are the difficulty of implementing this and having OpenGL ussage at the same time as well and of course the difficulty of working with any Microsoft API when you are not paying them trough the nose for development support (and even then thing are not easy).
The first these two problems could potentially be resolved by moving to DX completely which would make sense on Windows but which would likely mean having to fork XBMC.
The remaining difficulty is how on earth the DX API can be made to offload the video decoding to the GPU. Other open source group(s) seem to have been able to get this working, which means that the information on how to do this is out there. So the XBMC team should/might be able to extract the secrets from the code and or the developers that implemented this in the oter projects.
The main question remaining is if it is worth it splitting XBMC in an attempt to make a Windows version with GPU acceleration for video decoding...
As mentioned before even low end CPU's are powerful enough to do 1080p decoding with windows running so the need to use a GPU for this is getting les and less. Of course for the old P3 and P4 machines that start to sweat when one throws a DVD at them let alone a nice 720p movie these will not be able to playback a 1080p movie. But in all honnesty if an XBOX is not able to do it and people are willing to accept that why should extra efforts be made to have a P3 do this?
Of course if you use less CPU power to do this then you can always do other things with that head room, but lets be honnest how many people are running multiple processor intensive applications in behind their full screen 1080p movie, on a windows box... exactly.
Though GPU based video acceleration would look nice and advanced, looking at the power profile of a GPU compaired to a CPU I would not be surprized to hear that a GPU uses more power when decoding a video stream then a CPU does. So all in all honnesty I would much rather see more sound fixes, improved subtitle handeling, the inclusion of media flags and other misc information in the XBMC database and so on, basically stuff that we can all enjoy rather then expending a huge amount of effort on getting GPU video decoding working under Windows.