The Rumor Mill teasers that GW has been putting out are said to be a Grey Knights backpack, and these pics tend to point prove that point. Check out these images sent in by a reader.
via a reader on Faeit 212
Recently bought a grey knight terminator box. And I believe it is very obvious that the latest rumour image from GW is a GK of some form.
Below is a image of one of the torso backs from the sprue, which is identical to the rumour image (excluding what appears to be a bit of a cape)