I first met Paul Walsh in the early 2000s, back when he was a very visible presence in the London tech entrepreneurship scene. Three or four years ago, though, he seemed to vanish, or at least lower his profile dramatically. Every time I saw him he spoke in cryptic terms about a big project that he was working on. I’d find out soon enough, he assured me.

In January of this year, I ran into Walsh and his wife Sheetal Mehta (a former director of VC Relations at Microsoft) in San Francisco. They had just relocated to the city from London and were finally getting ready to launch their company. I made them promise they’d let me share the full details first on PandoDaily (a heck of a promise given that PandoDaily had only just launched). Finally, yesterday afternoon, Walsh made good on that promise.

The company is Metacert, and Walsh and Mehta have just closed a $740,000 funding round from a dozen angel investors, spanning the globe from Silicon Valley to New York to London, Russia and Dubai. Their aim? Nothing less than ridding the Web of porn, at least for those who want it gone.

In the video below, Walsh explains how the current solutions for filtering adult content are broken. Many are keyword-based, blocking innocent sites and allowing a few really bad ones to slip through the cracks. Others use outdated databases or clumsy rules. Metacert’s database, which mixes automated spiders and real human intervention, has cataloged and categorized 600 million pages of adult content (see screen grab above). Thousands more are being added daily. Walsh claims that the way the data is structured vastly reduces false positives, while allowing false negatives to be corrected immediately.

But obviously none of this data is useful unless users have access to it — and that’s where Metacert’s plans get ambitious. In addition to licensing its data to ISPs, hardware manufacturers, and corporations for use in their own filtering technology, the company is developing its own suite of consumer-facing filtering tools.

The first is a simple browser plug-in, which Walsh admits is more of a proof of concept than a finished product. In the next “four to six weeks” though, Metacert will launch its one-click filtering app, which allows parents and other moral guardians to disable all adult content with the flick of a (software) switch. As Walsh explains, the app works by changing the user’s DNS settings, making it browser agnostic. “It just works,” he says. Power users will be able to change their DNS settings directly, using what Walsh describes as “a kind of OpenDNS on steroids, for porn [filtering].”

(Update: Walsh just emailed to say the DNS features are ready now, and Metacert is looking for 50 beta testers — email beta@metacert.com if you’d like to be one of them.)

There’s already interest in Metacert’s technology from corporations and government bodies on both sides of the Atlantic. Metacert recently advised a British parliamentary body investigating whether the UK should implement a nationwide opt-out porn filter (Walsh’s advice: no. Filtering should be opt-in, otherwise it’s censorship). In the US, the list of companies Metacert is either working with or in advanced discussions with is certainly impressive (but, annoyingly, off the record).

Also secret is the product that Walsh hinted at, when I mentioned the problem of parents wanting to hand their iPads over to young kids, but paranoid they’d accidentally open Safari and quickly be knee-deep in smut. I suggested that a logical next development for Metacert would be a full-blown family-safe browser (I also suggested the name “Dora the Internet Explorer” — that one’s a gift, Paul). Walsh refused to confirm or deny that such a product was in the pipeline. Okay, fair enough.

What is for sure is that with both the British Prime Minister and US presidential hopeful Mitt Romney both talking up the idea of filtering Web content, the porn-blocking industry could be about to get a lot bigger. And a lot of smart people are betting that Metacert has at least some of the answers: JP Rangaswami, former chief scientist at British Telecom, has just become the company’s chairman while Vijay Tella (Qik, Oracle) and Bhaskar Roy (ditto) have respectively become a board member and board observer/investor.

Here’s our video interview. Sorry about the noise. Mission coffee shops are not the best place to record audio, it turns out.

Paul Walsh — Metacert from Paul Carr on Vimeo.