optional CAPTCHA fallback

i've been trying out Mollom, but i don't really like their terms of service, so i'm back with Akismet again. one thing i do like about Mollom, however, is their CAPTCHA fallback. i'd love to see this feature added to the akismet module as well:

if a comment/post/etc is flagged as spam, the user is (optionally) given a notice "It looks like you're spamming this site. Please fill out the following CAPTCHA to prove you're a human...", and is given a second chance. i imagine one of the current captcha modules could be used to add the captcha functionality...