At Legal Workshop, Richard Nagareda (Vanderbilt) has a shorter version of a full-length NYU Law Review piece (PDF) on the procedural history of class certification:

With so much riding on the class certification determination, one would have thought that procedural law would have arrived quickly at a clear and broadly shared understanding of the nature of that determination and the permissible parameters for inquiry by the court. That, however, has not been so. ...

The hard questions surrounding class certification today are--contrary to conventional wisdom--only superficially questions of fact, conflicting evidence, and dueling expert witnesses. Properly understood, aggregate proof frequently offers not so much a contested account of the facts that bear on class certification but, more fundamentally, an implicit demand for a new and often controversial conception of the substantive law that governs the litigation at hand.