A friend of my built a CAPTCHA generator out of his frustration with commonly available ones being easily circumvented. As a test, he has issued a challenge to see if his generator can be defeated.
My gut tells me that it can be defeated, but I hope that I'm wrong. If it ends up being highly successful, I would love to see some kind of proof-of-concept, person-to-person (non-automated) email protocol built around it. This would absolve the need to have all emails digitally signed - only the automated ones would need it.