A real life mechanism is the only thing we could think of too. Send a postcard to supplied snail-mail addresses, and require a code on that postcard. And while it is possible to get multiple snail mail addresses, it requires a lot more effort that email addresses.

As for the web-of-trust, I'm not sure how it would work? If I am a member (because someone knows me, say), how do you know that members that put me as their referrer are real people, and not me?

Just having me saying that "yah, I've met Joe", doesn't prove to anyone that I'm not lying, and I'm really Joe.

-- zigdon