From the press page at Nikon:
Nikon Corporation (KIMURA Makoto, President, Tokyo, hereafter referred to as “Nikon”) filed a patent infringement suit against Sigma Corporation (hereafter referred to as “Sigma”) to Tokyo District Court on May 25, 2011. Nikon’s lawsuit seeks an injunction against Sigma’s manufacture and sale of infringing interchangeable lenses with vibration reduction for single lens reflex cameras, along with damages for past infringement.
Nikon has invested considerable resources in the form of research and development over a period of many years. Such investments have created intellectual property that are important business assets of Nikon and have enabled Nikon to continuously provide the products and services that satisfy its customers requirements.
Although Nikon attempted to resolve Sigma’s patent infringement through negotiation, a non-adversarial resolution could not be reached. Consequently, Nikon concluded that filing a lawsuit was the only way it could protect its intellectual property.
Thanks Marty for the Heads Up! Maybe now we know why Sigma is charging so much for their new Sd1 DSLR!