] This is the homepage of the Shape Contexts based approach
] to break Gimpy, the CAPTCHA test used at Yahoo! to screen
] out bots. Our method can successfully pass that test 92%
] of the time. See EZ-Gimpy in action at Yahoo! The
] approach we take uses general purpose algorithms that
] have been designed for generic object recognition. The
] same basic ideas have been applied to finding people in
] images, matching handwritten digits, and recognizing 3D
] objects.

Nice.

So, any technological effort to prevent bot attacks is going to be easy to defeat. This may actually push demand for a federated identity system for the internet. The people managing systems will be responsible for verifying that their users are real people. An easy way to do this is to require telephone authentication (the way GeoTrust does for their personal SSL certificates) and not offer more then 10 addresses per phone number.

There are also interesting questions here about reputation carrying over between communities. You might need to gain a large audience on, say, MemeStreams before you are allowed to join another community with a more controlled atmosphere...

Breaking a Visual CAPTCHA