As mentioned by Kano this morning in the Phoronix IRC channel (#phoronix on freenode.net), there's now a snb-encoder Git branch for libva, the VA-API library. As of three days ago, the Sandy Bridge encoder was committed to VA-API's i965_drv_video driver.
In particular, this is the big commit. The encoder adds in over 2,000 lines of code to the VA-API library and is principally authored by Intel's Zhou Chang. Some of the code that's only being publicly pushed out now dates back to 2009.
The initial commit was limited to intra-frame encoding with Sandy Bridge hardware but another commit now seems to have inter-frame encode too. Older Intel graphics like Clarkdale / Arrandale have VA-API decoding support already, but there is no encoding support.
While the VA-API user-space code is out there, multi-media applications still need to take advantage of the VA-API encoding API for it to be useful. Many applications take advantage of VA-API decoding already since it's supported by various drivers for many months, but I am not aware of too many applications (any?) that yet target VA-API encoding (if you know of any, post in the forums). Obviously though Intel has found the support somewhere or has created out-of-tree patches in testing their SNB video encoding implementation, such as with the results shared yesterday.
Look for official Sandy Bridge VA-API video encoding to be announced for Linux sometime soon.