The consequences of the ruling will affect millions of people hitherto beyond the reach of US law, because 60 per cent of the material disseminated on the worldwide computer network - accessible by anybody, anywhere, who owns a PC, a modem and a phone line - originates inside the US. It is estimated that 35 million people in 160 countries have access to the Internet.
The act, passed overwhelmingly by Congress and signed in February by President Bill Clinton, makes it a crime to transmit sexually offensive material which may be accessible to children, extending the Internet definition of what is offensive beyond limitations imposed on printed matter.
A coalition of more than 20 organisations - including Internet service providers such as America Online and Compuserve and the American Society of Newspaper Editors - responded swiftly when the law was introduced. On 26 February they filed suit to challenge the act's constitutionality in terms of the First Amendment, an 18th-Century creation which prohibits Congress from "abridging the freedom of speech". Since then the law has been put on hold, pending the judges' verdict.
Brock Meeks, a political commentator on an on-line magazine called Hotwired, is one of the plaintiffs in the suit. "The people behind the law want to equate indecent speech with obscene speech," he said. "Indecent means bad language. Obscene means, for example, child pornography. There are already old laws in place which restrict obscenity. What these guys want to do - the Justice Department explicitly said it - is define the Internet as a broadcast medium, which in the US is heavily restricted because TV, unlike the Internet where you have to seek out material, is all-pervasive."
The court hearing, the first held to determine the First Amendment standards that should be applied on the Internet, began in Philadelphia on 21 March with the judges receiving a crash course on the mysteries of cyberspace travel. Closing arguments ended a month ago.
Leading the plaintiffs' coalition is the American Library Association (ALA), which represents 80,000 public libraries, many of which are making books available on-line. The association's chief complaint is that the definition of what constitutes indecency in the context of the act is too wide, because it seeks to apply the same standards to the Internet as are applied to broadcast media. Libraries could face criminal sanctions for making books available on-line which in print, where the legal limits on indecency are much broader, conform to US law. The new law, the ALA has said, narrows what is available on the Internet to what is deemed suitable for young children.
Lady Chatterley's Lover and Joyce's Ulysses, for example, fall clearly into the category of books that the Communications Decency Act forbids on the Internet. Texts with swearwords commonly used by adults in everyday speech will also be banned if the Philadelphia judges uphold the act. Newspaper and magazine editors are unhappy with the new law, because as more and more publications offer on-line services they may find themselves liable for prosecution unless they re-edit articles before putting them out on computer.
Human Rights Watch, a US-based organisation, has criticised the law because it stands up for free speech, but also because of fears that it could itself be found in violation.
Bruce Ennis, the lawyer who has been leading the appeal against the act, believes the basic flaw in the government case is that legislation to regulate indecency on the Internet will hurt the innocent while leaving the door open to the pornography peddlers it is seeking to restrict. "One of the arguments we've been making that is overlooked by the federal government is that we are talking here about a global medium of communication. So far, 40 per cent of host computers are located outside the US, but that number is growing. A domestic law, therefore, is going to be futile."
Besides, Mr Ennis said, the technology already exists to allow computer users in America to send out material on the Internet while creating the impression that it is being sent from abroad: "Anonymous remailers is the name of the system. It allows you to reflect the host site in another country. The system's virtually untraceable, sort of like a Swiss bank account."
All of which presents advocates of the Communications Decency Act, like Donna Rice, of an anti-pornography organisation called Enough is Enough, with a seemingly impossible conundrum. Ms Rice, who rose to prominence a decade ago when the news broke that she had had an affair with presidential candidate Gary Hart, was among those who worked with Congress to frame the act in the first place. "We became involved when we saw the proliferation of child pornography on the Internet avenues. Pornographers were provided suddenly with a new window of opportunity, and we wanted to close it."
Ms Rice agreed, however, that laws alone were not sufficient to deal with the problem. "We need a three-pronged approach. As well as the law we need to develop technological solutions such as parental empowerment software."
But what about the difficulty, as Mr Ennis had observed, of imposing restrictions on a network that was global? "OK. We need a fourth prong. We need international co-ordination to deal with this." Like the United Nations? "Yes." Ms Rice refused to be beaten into submission, but she was obliged to acknowledge that the dragon she was trying to slay was multi-headed: to overcome it would require an almost superhuman effort.
Either that, or the computer boffins will have to come up with a fool- proof censorship mechanism. Albert Vezza, associate director of the Massachusetts Institute of Technology's laboratory for computer science, told the Philadelphia court in April that a system would soon become available to enable parents to impose ratings restrictions, as the film industry does, on Internet sites.
How such a system, or variations thereof, is practically applied would ultimately depend on the piecemeal judgement of parents, or indeed governments. But the question of what constitutes indecency on the Internet is not amenable, in Mr Ennis's view, to blanket legal interpretation, suggesting that the Communications Decency Act was cobbled together by people who lacked a full understanding of how the Internet works.
If the judges rule that the act is unconstitutional, Mr Ennis said, "it would mean the government would have to go back to the drawing-board and start all over". Whatever the outcome, one side or the other is expected to lodge an appeal before the US Supreme Court.