Nikon has sued Sigma over patent violations related to VR (Sigma's OS). Japanese companies don't tend to be litigious. There's a lot of behind the scenes negotiating and patent swapping that happens, but it's much more rare in Japan for one company to sue another than it is in the US or even Europe. But today Nikon announced that they were doing just that. Apparently negotiations to reach a settlement went nowhere, and now Nikon is seeking an injunction to stop all shipments of Sigma OS lenses.
As long as Sigma was primarily making Nikon and Canon lenses, I somehow doubt that Nikon would have taken this step. But with the prospect of Sigma lenses for NEX and m4/3 mounts, if Sigma really is infringing on Nikon's patents, it makes perfect sense for Nikon not to let that technology propagate into their competition.
The gloves are coming off in the camera industry. Coopetition, that unique form of collaborating and competing with a rival firm, is more and more giving way to straight out competition in Japan. That bodes well for the vertically integrated companies (Panasonic, Samsung [Korean], and Sony). It bodes less well for the non-integrated companies (Nikon, Olympus, Pentax, Sigma).