Intel has been working on the Haswell open-source Linux enablement for months from compiler optimizations to open-source graphics acceleration code for enabling support for the Ivy Bridge successor as well as future Intel motherboard chipsets for Haswell. Recently they pushed Haswell video acceleration code for exposing VA-API support for hardware-based video acceleration playback on the integrated graphics processor.
The patches reveal that Haswell introduces "VECS", a new command streamer that allows offloading of video post processing to another new component, VEBOX. The VEBOX engine is used for the hardware-based video post processing with supported user-space code. Tuesday's patches touch a few hundred lines of code within the Linux kernel's Intel DRM Driver for supporting VECS/VEBOX.
This work will hopefully be ready and merged for the Linux 3.8 kernel but for now can be found on the intel-gfx list. It's looking like the Linux 3.8 kernel will be the sweet spot where the Haswell Linux support all comes together nicely after previously they admitted they screwed up with Haswell support.
With the future Intel hardware now having a video post-processing engine, it looks like Intel will continue to be the winner for Linux enthusiasts interested in open-source video playback for HTPC and other purposes. Their entire video stack is fully open-source while AMD has yet to document or provide code concerning their UVD video decode engine and the Nouveau developers are still working on their reverse-engineering and support for NVIDIA PureVideo.