From what I have read, Sony's sensor division is probably bigger than all of Nikon. They have the $$$ for R&D and I'm sure Nikon pays a premium. Sony has the deepest pockets of the the 3 and Canon is rumored to begin using Sony sensors also, they are probably tired of being left in the dust in the sensor area.
Sony is about to take the mirrorless world by storm. Once they have significant market share, they could start retaining the best/latest for their own models. At the moment, Nikon is paying their bills. They are already significant shareholders in Olympus and now have the best in body stabilization tech in their upcoming bodies, cloned from the Olympus EM-1. This 5 axis stabilization is superior to Nikon's VR and probably makes for more efficient lens manufacturing. I could see them picking up the Nikon brand down the road. If that happened, Canon may find themselves in a very uncomfortable #2 position.