I'm very sorry to report, via Larry Solum, this shocking news. At the time of his death, he held an endowed Chair at the University of Illinois College of Law, where he had taught since 2002. He was well-known for his wide-ranging work on issues in corporate law and securities regulation. I will post links to memorial notices when they appear.
UPDATE: Professor Ribstein suffered a massive stroke yesterday, and passed away earlier today.
ANOTHER: Several warm remembrances and tributes here (and scroll down).
AND ANOTHER: The memorial notice from the University of Illinois College of Law.