Integrate with spam module (http://drupal.org/project/spam)
Check all the textareas for spam during form validation and if one triggers,
set a form error and inject a CAPTCHA as a second step to the form.
I know Mollom does something similar, but Mollom is way overkill, because it requires an account on an external site, and has trusted users, which we don't really need etc, etc. AND Mollom cannot be used with any form like CAPTCHA can.