I don't get why this is more than academically interesting.
Spammers are willing to spend money on humans to solve trivial problems. CAPTCHAs should only be one element in a broader set of protections.
As an example, Yahoo financial message boards. No regular user registers, then immediately proceeds to post 100 messages to 100 different boards separated by only a few seconds. A CAPTCHA per posting won't block this spam. Analyzing behavior will quickly shut it down.