Brilliant

Conditional CAPTCHA is a great solution to weed out false positive results from Akismet and finds a great middle ground between CAPTCHA (which can annoy normal users) and effectively blocking spam. With Conditional CAPTCHA, only Akismet-positive comments are presented with a CAPTCHA (or reCAPTCHA), so normal users don’t even have to bother with entering a CAPTCHA most of the time. Simply brilliant, our moderation queue went from 500 per day to 1.

Note that you need to disable other comment plugins, especially Hashcash, as they conflict and Hashcash takes precedence! But no worries, you won’t need it anymore.