Signed up for cell-phone service lately?

If so, maybe you remember the part where you sat down with representatives of your prospective carrier - let's say it was AT&T Mobility - and hammered out terms of a contract.

It was just you and AT&T at the bargaining table, unless you wisely brought your lawyers along to help. These negotiations can get pretty tricky, and AT&T surely had its high-priced legal team on hand.

You say you don't remember that?

OK, I confess: That scene was imaginary. But here's the rub: An alternate universe where it occurred would be about the only place last week's U.S. Supreme Court ruling favoring AT&T in a class-action case makes much sense.

Once again, a sharply divided Supreme Court issued a 5-4 ruling that seemed to be more about the pro-business results it wanted - in this case, slamming the door on consumer class actions - than about the legal arguments behind them.

And once again, supposedly conservative justices who claim to care deeply about federalism were willing to stretch a law well beyond Congress' intent and to tell one of the 50 states that it can't apply its own statutes and rules.

Before I explain further, a little history.

The Concepcions, the California couple who filed the case, went to court contending that AT&T misled them when it billed them $30.22 for a cell phone that was supposedly free. Because there was reason to believe the charges were widespread - the carrier said it was simply assessing sales tax for the actual cost of a subsidized phone - their lawyers filed the case as a class action.

AT&T fought them on procedural grounds: It said the Concepcions had no right to be in court at all, because their contract with the carrier required that disputes be resolved via arbitration. And it said they also had no right to claim that other customers were similarly harmed, because the Concepcions' contract with AT&T barred them from joining in a class action - in court or before an arbitrator.

AT&T's solution? Customers who felt wronged could try to get their money back by filing individually for arbitration. And the company touted its unusually consumer-friendly arbitration process, under which a successful claimant could get lawyers' fees and even a bonus award of $7,500 - if the arbitrator found he or she was entitled to more than AT&T's final offer.

Ruling in AT&T's favor, Justice Antonin Scalia said the case turned on a 1925 law, the Federal Arbitration Act, that he said preempted California state law. "Arbitration is a matter of contract," Scalia wrote, "and the FAA requires courts to honor parties' expectations."

What's the problem with Scalia's ruling? Unless Congress intervenes, the court has essentially allowed any company that deals with consumers to mistreat them with impunity. By writing a contract that mandates arbitration and at the same time bars class actions, a company can insulate itself from the only kind of legal challenge - a class action - that can deal with a large number of small-dollar wrongs.

In his dissent, Justice Stephen G. Breyer explained why individual arbitrations, even under terms such as AT&T's, are an unrealistic substitute.

Breyer noted that AT&T could dodge the $7,500 bonus offered to an individual claimant simply by offering the full value of the dispute - in this case, just $30.22 - before an arbitrator is appointed.

Breyer reached back to an earlier appeals court case to explain what's really at stake: "The realistic alternative to a class action is not 17 million individual suits, but zero individual suits, as only a lunatic or a fanatic sues for $30."

The truth is that consumers have few good options when they believe they've been wronged for a small amount of money. Only the most aggrieved will bother to write an angry letter, complain to a government agency, or post their gripes online. Most of us won't bother filing for arbitration unless a larger amount of money is at stake, any more than we'd contact a lawyer to sue. But we benefit indirectly when class-action lawyers recognize that an individual wrong is part of a pattern, and take steps to challenge it.

Class actions are hardly a perfect solution, and both the courts and Congress have worked for years to refine them. But consumer lawyers can legitimately take credit for addressing problems such as unfair or deceptive bank charges, cable-television late fees, and telemarketing schemes - and, at the same time, deterring other companies from questionable practices.

Philadelphia class-action lawyer Michael D. Donovan says the underlying problem is the explosion of so-called "contracts of adhesion" - the kind of take-it-or-leave-it deals that the Concepcions were offered by AT&T, and that consumers face every day when they sign up for various services, open bank accounts, or even click "I agree" to a website's terms.

Donovan says that when the Federal Arbitration Act was passed, arbitration clauses were added to contracts during negotiations by parties of similar power who could anticipate future disagreements. "The 1925 act was there because shippers and diamond merchants wanted to arbitrate disputes about spoiled goods and misclassified diamonds," he says.

Modern consumer contracts are at the other end of the spectrum. Even if you brought your lawyers to the cell-phone store, you'd still be stuck with AT&T's take-it-or-leave-it terms. And as a cell-phone customer, you wouldn't have many alternatives. You could choose among three other national carriers - or just two, if AT&T is allowed to take over T-Mobile.

Before last week's ruling, consumers at least could band together to challenge the fairness of those terms, or of how companies applied them. The Supreme Court's ruling is an invitation to companies to follow AT&T's lead and box consumers out of court.

"What the court has said is that corporations have the power to impose whatever terms they want to impose as a condition of doing business with them," Donovan says.

And one of those conditions can be that you don't have a legal right to go to court to challenge any of the rest.