Initial support was committed yesterday and since then there's been lots of Broadwell-related commits to VA-API. On the video acceleration side, there's lots of changes between Haswell and Broadwell.
More of the Broadwell VA-API commits can be found via VA-API on Anzwix.
Intel Broadwell Linux graphics support is in good shape with Mesa 10.2 and can work out okay on Linux 3.14 but more improvements are coming to Linux 3.15. The support should arrive fine though since Broadwell isn't expected to be released for a couple months.