Alex came up with an idea a while back to try and get tear free EXA and Xv rendering. Tear free Xv means we hopefully can get textured video on (r100->r500) without ugly tears in the middle. Pierre Ossman took on the proof-of-concept Alex hacked together and spent some time making it all work properly. While he was there he picked up the r3xx/r4xx bicubic shader two of the other developers (Maciej and Corbin) had started but never had time to finish debugging, and pushed it forward so it works on those chips.
Its always good to know there are developers out there scratching itches and that with a small bit of input from Alex or myself can push the project to do what they want. There have been many accusations levelled at the radeon codebase over the years but the number of really useful community contributions has risen a lot lately, so it can't all be bad!!
So if you want tearfree Xv video on your r300/r400/r500, radeon is now the only driver that can provide it.
No we don't have any r600 support for Xv yet, stay tuned hopefully :)