The Ledbetter case centered on a claim of intentional discrimination. Lilli Ledbetter, who worked as a supervisor at Goodyear's tire plant in Gadsden, Alabama, from 1979 to 1998, alleged that several supervisors had given her poor evaluations, which led to lower pay increases than those of her male colleagues.

But the Wal-Mart suit alleges that the company engaged in a broad "pattern and practice" of discrimination over time, culminating in an "adverse impact" on women who worked there for many years.

"The Supreme Court hasn't answered the question on what the standard is in a pattern and practice case," said Brad Seligman of the Impact Group in San Francisco, the public interest law firm that represents the Wal-Mart women.

The central issue in the Wal-Mart case has been whether the group could stand together as plaintiffs in a class action, given the sheer number of women who have worked in the stores.

In February, a federal appeals court in San Francisco affirmed a lower court ruling that they could proceed to trial. Wal-Mart moved to challenge that ruling with the entire panel of judges on the United States Court of Appeals for the Ninth Circuit, which hasn't yet announced whether it will take the case.

While the Ledbetter decision does not, by itself, derail the Wal-Mart suit, Seligman said that in a "worst-case scenario" the ruling might limit the plaintiffs' time frame for collecting damages. The class action goes back to December 26, 1998, but some claims could be based on pay decisions made prior to the beginning of the class period.

Richard Samp of the Washington Legal Foundation, which has filed friend of the court briefs on behalf of Wal-Mart, expects as much: "It could well be that certain members of the class will have their claims time barred. There are already people talking about that," he said.