Mollom is based on the idea that if we are not 100% confident about the classification of your content in the ham or spam group, we present a CAPTCHA to the user. This improves the classifier's performance and eliminates the need of a moderation queue. And because these CAPTCHAs are only shown in the "gray zone", only a very limited number of real users have to fill in a CAPTCHA, while most of the CAPTCHAs are actually shown to spam-bots.
Here is actual data from the Mollom server showing the number of requests of the different types:
This plot shows Mollom's server statistics on any particular day, showing the ham messages in green, the spam messages in orange, and the unsure messages (where a CAPTCHA was shown) in gray. Also notice the thin brown line just on top of the ham region (look close, it is barely visible :), which denotes the human users that had to fill in a CAPTCHA.
In numbers it boils down to this: on average 80% of all content is spam of which about 40% is shown a CAPTCHA and 60% is directly blocked; from all the user generated ham content, which is the remaining 20% of the whole, only 5% (of this 20%) was seen as unsure and had to fill in a CAPTCHA to get accepted. Not too bad... And thanks to both the image and audio CAPTCHAs, there are no accessibility issues.