http://groups.google.com/group/twitter-development-talk/browse_thread/thread/c4820fbb9bb296d7
BTW, I've been using the term "CAPTCHA" incorrectly. It stands for "Completely Automated Public Turing test to tell Computers and Humans Apart." What I describe here isn't automated. Users must submit a question to test comprehension of their tweet selection. But like a CAPTCHA, we want to make sure this is not a computer (or mindless clicking by a human).