I have not found time to develop my approach to a next-generation CAPTCHA any further (here is my original outline of the idea from last year). I still think that a collaborative process of creating and solving interesting and/or creative riddles is a really good idea, so I want to at least put this idea out there. Here is the presentation I developed for it together with Kathrin:
Also, I had started a website for this project before I ran out of time. Here are some blog posts.
Update 10.03.09: The editors at slideshare.net featured this presentation on their start page. Thanks! Btw, this list of featured presentations is a great place to shop around for interesting and well-digestible concepts.