When a plaintiff invokes the federal courts' diversity jurisdiction, it must plead both the state of incorporation and the principal place of business of corporate parties. In 2010, the U.S. Supreme Court in Hertz Corp. v. Friend adopted the "nerve center" test for determining a corporation's principal place of business. Earlier this month, the Ninth Circuit held in Harris v. Rand that, while Hertz provided a uniform test for determining subject matter jurisdiction, it did not adopt a heightened pleading standard for the allegations supporting diversity jurisdiction. So it's sufficient for a complaint asserting diversity jurisdiction to state the location of a party's principal place of business; plaintiffs are not required to plead facts showing why that location is the "nerve center." But the Ninth Circuit warned that, if the allegations of a principal place of business are "implausible," then the District Court has the authority to require more specific pleading.
See our earlier coverage of Hertz here, and our coverage of Bell Atlantic Corp. v. Twombly and Ashcroft v. Iqbal, the Supreme Court's key opinions regarding federal court pleading standards, here and here.