So, dante76 posted this on warseer:
Through a new source, who with hindsight turned out to be pretty accurate with Blood Angels and Skaven....
-Pencilled in release date of Aug 2011
-New Alien allies
-All metal sets to plastic (Pathfinder, Krootox, Vespids)
-Possible new tank
-Changes to Hammerhead Railgun rules. Essentially draws a line across table hitting everything in line. Multiple pen through vehicles, only stopped by a glancing hit.
-Changes to markerlights. Point system. 1 Markerlight point = + 1BS, 2 Markerlight points = reroll to wound [or] hit 3 = Difficult Terrain test (blinded by the lights?). Cannot be stacked. So can't have reroll to wound and reroll to hit at 4 points.
-Battle Suits. A plug and play system. Essentially streamlined. Pick two guns and a skill or two skills a gun. New models might have interchangeable arms similar to Killa Kans
Well, we know about the Dark Eldar and Grey Knights being close to finished. The Necrons keep phasing out then making their well be back saves, and are expected soon. Which would leave Witch Hunters as the only 3rd edition codex. Black Templars and Tau are the oldest 4th edition books.
So this does fit in very well with both an Imperial/Xeno flip-flop release set up (which would be a welcome change in release style) and for updating all the oldest books.
The potential rules seem to fit the trends of newer releases.
Railguns moving through objects (blood lance, jaws of the world wolf)
Markerlights being a tally system (tally of Nurgle, the dark eldar slavers rumor, saga of the warrior borne).