An academic study found corporate boards are more likely to be influenced by the recommendations of equity analysts following the 2002 rule change that separated the analysts from investment bankers. The study conducted by professors at the Paul Merage School of Business at the University of California in Irvine and at the Jesse H. Jones Graduate School of Management at Rice University in Houston, found that this increase in trust in analysts meant that boards are more likely than in the past to fire an under-performing chief executive based in part on analyst recommendations.The paper is titled CEO Dismissal: The Role of Investment Analysts as an External Control Mechanism, and it's authored by Margarethe Wiersema (of UC-Irvine) and Yan Zhang (of Rice University). It's a pretty good example of the way that regulatory changes affect the impact of various monitoring agents. My take on it is that post SOX, boards are much more likely to "yank the cord" on CEOs following a whole host of "bad news" events (earnings disappointments, product recalls, etc...). I bet that'd make for an interesting research topic for someone (offered free of charge - I'm not going to pursue it).
You can read a PDF of a working paper version of paper here.