A unanimous Supreme Court in Hertz Corp. v. Friend adopted the "nerve center" test today for citizenship under 28 U.S.C. s1332.
http://www.supremecourtus.gov/opinions/09pdf/08-1107.pdf
Basically this means that plaintiffs in California can't avoid federal court by arguing that the proportional percentage of a corporation's business in California is so large (since California is so large and populous) that the corporation is a California citizen. So a corporation does still have some control over the identity of the state of which it will be deemed a citizen.
