HALF MOON BAY, Calif. – Tech visionary Lawrence Lessig made a sobering prediction Tuesday at Fortune’s Brainstorm Tech conference: “There’s going to be an i-9/11 event,” he said, “an event that demonstrates the instability of the Internet, and that inspires the government to a response.”

He said he believes this digital disaster – a major hacker attack or other act of cyber-terrorism in the next 10 years – will prompt the U.S. government to clamp down on Internet freedoms in an online parallel to the Patriot Act.

Stanford professor Lawrence Lessig predicts that an online attack will prompt the government to clamp down on Internet freedoms. Photo: Gary Wagner

Lessig, a Stanford Law School professor who founded its Center for Internet and Society, said he came to this conclusion after a conversation with former federal counter-terrorism adviser Richard Clarke. Lessig said Clarke told him that the Justice Department had already written up much of the Patriot Act before the terror attacks of September 11, 2001, and that there is a similar proposal on the shelf in case of an Internet catastrophe. Advocates of Internet openness will not be thrilled about its contents, Lessig said. “Vint Cerf is not going to like it very much,” Lessig recalls Clark saying.

It just happened that Cerf, and Internet pioneer who now works for Google , was in the audience. And the warning obviously got his attention.

The prediction was part of 2018: Life on the Net, a panel with tech thinkers Lessig, Joichi Ito, and Philip Rosedale. Conversation topics ranged from copyright policies to virtual worlds to mobile economies. The panelists were generally optimistic about how the Internet will develop – Rosedale, founder of virtual world Second Life, made an unsurprising prediction that virtual environments like his would comprise the majority of Internet traffic in a decade, for example. But their discussions about the potential pitfalls were a bit more entertaining.

Entrepreneur and venture capitalist Joichi Ito said would-be investors in mobile technology should take a warning from Japan’s market. Photo: Gary Wagner

Ito, an entrepreneur and venture capitalist, had some words of warning for others investing in mobile businesses. In today’s fixed-line Internet, he said, the money that companies make from their ideas tends to get plowed back into the innovation economy, seeding startups like Facebook and Twitter. In Silicon Valley’s virtuous cycle, successful entrepreneurs and companies tend to keep the money circulating.

But no one should assume mobile will work the same way in the U.S.: Just look at Japan, where the mobile Internet is big. There, just as here, the wireless carriers who own the networks want a piece of the transactions that happen on their networks. The result is that in Japan, carriers have taken a sizable chunk of the spoils from successful ideas and sunk them into their own “bloated R&D labs,” Ito said.