A: 500.

Members of the cybersecurity panel believe we live in a world pervaded by malicious digital intrusions by criminals, hacktivists and governments. We keep building higher walls, but hackers and industrial spies always find ways to leap over them.

DanMcGahn, CEO of American Superconductor, which did a brisk business selling software to Chinese wind turbine makers, told of the illicit sale of vital code that led to a revenue slide and the collapse of the company's stock. McGahn's story illustrated the potentially widespread impact such crimes could have on U.S. jobs, said Michael Chertoff, chief of Homeland Security in the Bush administration.

Now a security entrepreneur, Chertoff spoke about the common notion that hackers want to pilfer credit card numbers, when the greater threat is that they may live in your corporate network, hidden, for years. And he marvels at how easy social media makes the task for Internet miscreants. Facebook can tip them off to your whereabouts. LinkedIn gives away too much information about corporate personnel.

Concerns are rising that the problem will invade the realm of geopolitics. Mikko Hypponen, chief of research for F-Secure, hopes we are prepared for nation-vs.-nation cyber-sabotage, and inevitably, cyberwarfare. (Hypponen was the source of that alarming factoid above about the Fortune 500.)

There's no encompassing solution. These experts urge firms and governments to intensify their vigilance and set up systems to limit the fallout when they're attacked. Also, they should rethink their assumptions about trust.

The bottom line? Trust and verify. Then verify some more.