In what turns out to have been a mistake, an "Q1 RC2 test report for SNB H264 encoding driver" email was issued to the intel-gfx mailing list. The email was written by Hai Lan at Intel. When he realized he sent it to the public list, he immediately recalled it, but it's now out there for the public to see anyways thanks to the open-source mailing list archives.
This email is regarding the H.264 video encoding support for Sandy Bridge on Linux. Unfortunately it looks like the results show the performance missing its target, which is perhaps why the code is not yet published and out there. Their "RC1" performance was at six frames per second, with "RC2" there are up to 104 FPS, which is a huge improvement. However, these internal results show the Windows encode driver at 516 frames per second. That's a huge difference.
Sadly, no other details were leaked out. Hopefully the necessary code will be published soon. It will require at least patches to the VA-API "libva" library, but it's not been officially said whether any DRM updates will be needed for supporting the SNB encode functionality. Hopefully it won't otherwise they won't be merged until the Linux 2.6.40 kernel.