Yesterday the United States Supreme Court clarified that a company's principal place of business, for jurisdictional purposes, is the "nerve center" from which the officers actually direct, control, and coordinate the activities of the corporation. In so doing, the Court threw out jurisdictional tests such as that in the 9th Circuit that analyze the quantity of a corporation's business within a state as compared to operations in other states.
For the opinion:
http://www.supremecourtus.gov/opinions/09pdf/08-1107.pdf