The Chicken Littles of the plaintiffs bar were pretty sure the sky was falling last year when the U.S. Supreme Court issued its ruling in AT&T Mobility v. Concepcion. In Concepcion, you'll recall, the Court upheld AT&T's right to compel consumers to submit to arbitration even though, under California common law, consumer class-action waivers were considered unconscionable. The ruling was seen as a broad endorsement of mandatory arbitration clauses and a simultaneous threat to class actions, for consumers and everyone else. If it weren't for Concepcion, for instance, would the Carlyle Group have included in its IPO registration statement a provision mandating that investors surrender their right to sue and instead submit to individual arbitration? I don't think so.

But plaintiffs' lawyers, regulators, and courts are beginning to poke some holes in Concepcion. In D.R. Horton, Inc. and Michael Cuda, the National Labor Relations Board weighed Horton's rights under the Federal Arbitration Act against its employees' rights under federal labor laws, and concluded Horton could not block collective employee actions through an arbitration clause.

The Financial Industry Regulatory Authority just sued Charles Schwab for adding a provision to customer account agreements that would preclude them from bringing class actions against the brokerage.

And perhaps most significantly, the U.S. Court of Appeals for the Second Circuit has concluded that Concepcion does not always bar class actions.

Wednesday's Second Circuit opinion in In re: American Express Merchants' Litigation finds tension between Concepcion and a Supreme Court decision from 2000, Green Tree Financial v. Randolph. In Green Tree, the Court conceded that the cost of an individual proceeding could stand in the way of a litigant exercising federal statutory rights through arbitration.

Though the Justices ended up concluding that the Green Tree plaintiffs hadn't shown that to be the case, the Second Circuit relied heavily on the opinion's consideration of individualized costs when it first decided the AmEx case back in 2009. Small merchants, the Second Circuit panel found, couldn't afford to challenge AmEx for alleged antitrust violations in individual arbitrations. (The finding was based on evidence from the plaintiffs' expert economist.) So according to the Second Circuit, AmEx's class-action waiver was an unenforceable bar on the plaintiffs' statutory right to bring federal antitrust claims.

AmEx came back to the Second Circuit for reconsideration after Concepcion. In Wednesday's ruling, Second Circuit Judges Robert Sack and Rosemary Pooler said that decision does not change their analysis, just as they previously found the Supreme Court's 2010 ruling in Stolt-Nielsen v. AnimalFeeds International didn't. (Justice Sonia Sotomayor was on the Second Circuit AmEx panel before her elevation to the Supreme Court.) "It is tempting to give both Concepcion and Stolt-Nielsen such a facile reading, and find that the cases render class action arbitration waivers per se enforceable," Pooler wrote. "But a careful reading of the cases demonstrates that neither one addresses the issue presented here: whether a class-action arbitration waiver clause is enforceable even if the plaintiffs are able to demonstrate that the practical effect of enforcement would be to preclude their ability to vindicate their federal statutory rights."

To be sure, the Second Circuit panel said that its ruling doesn't mean "that class action waivers in arbitration agreements are per se unenforceable, or even that they are per se unenforceable in the context of antitrust actions." But the opinion's conclusion that "each class action waiver must be considered on its merits" is powerful ammunition for class action lawyers.

AmEx plaintiffs' counsel Gary Friedman of the Friedman Law Group said the Second Circuit's opinion offers a "limited exception" to the rule that arbitration may pre-empt class actions. "This decision does flow out of well-established Supreme Court precedent and circuit court law interpreting [that precedent]," Friedman said. "It's on solid footing."

AmEx counsel Bruce Schneider of Stroock & Stroock & Lavan referred me to a company spokesperson who didn't get back to me. AmEx was also represented at the Second Circuit by Kellogg, Huber, Hansen, Todd, Evans & Figel.

(Reporting by Alison Frankel)