Professors Davidoff and Cain have published a paper indicating that over 97% of takeovers involving at least $100 million have been the subject of litigation. Professor Davidoff discussed his paper  in a post on DealBook, including reference to the recent comments by members of the Delaware Court of Chancery in response to many of these suits that settle based only on additional disclosures. Also, Kevin LaCroix on The D & O Diary provides an excellent overview of the paper.