What did the court decide in AT&T Mobility LLC v. Concepcion?
Answered for: US
Because it "stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress," Hines v. Davidowitz, 312 U. S. 52, 67 (1941), California's Discover Bank rule is pre-empted by the FAA.
The Court held that the Federal Arbitration Act preempted a California rule treating most consumer class-arbitration waivers as unconscionable. The sentence above is the court in its own words.
This answer is true in US and nowhere else on this site. Another jurisdiction is another answer, or an honest gap: see what has been read for US.