The California Supreme Court held many moons ago that a defendant may not defeat a class action by "picking off" the class representatives. La Sala v. American Sav. & Loan Assn., 5 Cal.3d 864 (1971).
The Ninth Circuit came to the same conclusion, based on essentially the same reasoning, in a decision handed down yesterday. Pitts v. Terrible Herbst, Inc., ___ F.3d ___ (9th Cir. Aug. 9, 2011).
The decision also contains some interesting language about the right to conduct discovery before moving for class certification. See slip op. at 10457-58 & nn. 4, 5.