A case that goes before the U.S. Supreme Court tomorrow, AT&T Mobility vs. Concepcion, may potentially ban consumers from filing class-action lawsuits. The basic question that will be decided is whether companies can bar class-actions in the fine print of their take-it-or-leave-it contracts with customers (and employees.)

The U.S. District Court for the Southern District of California ruled that a class-action ban violates state law and is not preempted by federal law; the U.S. 9th Circuit Court of Appeals upheld the lower-court ruling last year.

If a majority of the nine justices vote AT&T’s way, any business that issues a contract to customers — such as for credit cards, cell phones or cable TV — would be able to prevent them from joining class-action lawsuits. Class-actions allow plaintiffs to band together in seeking compensation or redress, thus giving more substance to their claims.

And the banning of class-actions may potentially apply to employment agreements such as union contracts…