This little tidbit comes from Blood of Kittens. It was such a small and rather bland rumor that I did not post it this morning, but here it is. Take all rumors with a grain or two of salt.
The Grey Knights are getting a new unit or they are getting an upgrade that causes an automatic hit to any enemy model in base-to-base with them. This hit wounds on a 4+ regardless of enemies toughness with normal saves allowed and occurs before ALL attacks (think old mandiblasters), but still adds to combat resolution at the end of combat.
This I could really care less about. It would hurt my dark eldar though, as we rely upon that almight I5.