Bulls in a china shop—or catalysts for change?  The divides created when activist investors muscle their way onto boards as varied as those of Hewlett-Packard, J. C. Penny, and Yahoo can run as deep as those on Capitol Hill. Has Ralph Whitworth of Relational Investors restored shareholder value at HP?  Did William A. Ackman of Pershing Square Capital Management destroy value at J.C. Penney?  Has Daniel Loeb of Third Point rebuilt value at Yahoo?