Pretty cool, definitely step in the right direction.

Is there ever a situation where a bunch of legitimate users have the same IP? I'm thinking at a university maybe, corporate firewall, or AOL?

Even so this compromise might be worth it to remove the login hurdle.

If you become Digg or something and people start actively trying to get around it, do you think this would be about as secure as logins? Could be a high quality problem to have.