If you care about music on the Net, you've probably been hearing this refrain a lot lately, repeated by MP3 traders, geek programmers and digital music industry types alike. On Monday in a courtroom in San Francisco, a judge will decide whether to uphold a preliminary injunction against Napster, potentially shutting down the music trading service -- but as a fallback, there's always Gnutella. Frightened by legal threats from the Recording Industry Association of America (RIAA), Scour may have laid off its staff and put its future in jeopardy -- but there's always Gnutella. And sure, the arcane file sharing software Freenet might not be ready for your average consumer yet -- but, of course, there's always Gnutella.

Is there, though?

Next week, Napster goes back to court to find out whether the service will be shut down for the duration of its trial. The Net is holding its breath in anticipation of the answer. If Napster is taken out, more than 30 million MP3 fans will surely flood the Net looking for a new home; Gnutella will probably be the first program many of those people download. Gnutella is not only already being heralded as the next Napster, but it's also considered by its most avid fans to be a better Napster: an open-source software program that is decentralized and anonymous, harder to sue than Napster and versatile enough to support all kinds of files.

Gene Kan, 25, Gnutella's lead evangelist and the man behind the Gnutella portal at gnutella.wego.com, believes that the software is prepared for widespread use, even if he admits that it currently is still flawed. "It was really clear to us from the outset that Gnutella software had a long way to go," Kan says, but he believes that most of the program's biggest problems have been solved: "Gnutella isn't perfect, but there's no huge, glaring thing missing." And, he says, "Gnutella is very popular; it's already very successful."

But according to critics, Gnutella is hardly ready for prime time -- and is facing dilemmas almost as worrisome as the Napster lawsuit. Over the last month, users of the system have noticed a dramatic slowdown in responsiveness, and a number of reports have revealed serious instabilities in the Gnutella network. The open-source software developers who nabbed the program after America Online forced its programmers to abandon it are still striving to learn how to work together. And Gnutella's legal status is also murky: The RIAA is already hinting that it may be preparing a strategy to attack Gnutella.

Defenders of the Net love to believe that "The Man" will never be able to shut down their decentralized, "peer to peer" (P2P) way of life. Their faith is not unreasonable. File-sharing programs (not to mention chat, e-mail and other means of shooting packets of information back and forth across cyberspace) are built into the fundamental structure of the Net, and will never be entirely eradicated. But it's also quite true that corporate America can still make things very difficult for would-be challengers.

Consider this: File-sharing systems work best when they reach critical mass -- only once they have a significant number of users is it likely that someone out there will have the file you want. That's why Napster has continued to grow; with 30 million users the odds are in your favor that one or two of them will have what you need. But as soon as a file-sharing system has critical mass, it's big enough and threatening enough to become the copyright protectorate's next legal target; and those file-trading masses are also going to strain the network to its capacity and beyond. That's the Gnutella paradox. The attainment of widespread popularity may in fact signal a file trading software program's imminent demise.

If the decentralized Gnutella can't handle the legal and technical threats that come from mass usage, what system can? Or are music traders doomed to confront a future in which each new "next Napster" is progressively undermined by its own success?