a3nm

I guess the problem is that it stops working when you start being big enough to attract the spammers' attention and get bots tailored to your registration system. In the meantime, I guess this is just to avoid problems with general stupid bots which just fill out all the forms they can't find without trying anything fancy.

Using OpenID makes the rest of the techniques moot (except for OpenID providers), right?

Yes, but they are only exchanging a constant amount of information to fool the server challenge, whereas we could hope to do better if the server builds challenges which use information that he knows the client has.

For some reason, this inspired me to write a blog post: http://a3nm.net/blog/deduplication_attacks.html and http://news.ycombinator.com/item?id=2489594