After making several revisions of the Drupal Captcha module, I've ended up with something I am satisfied with. My first attempt added support for comments and other content nodes and created a more complex image than the previous version. However, I was not satisfied.
Captcha images are not a very friendly solution for visually impaired users so I revised my captcha module to use a text based challenge response system. While I would agree that this could be circumvented by intelligent parsing (and I expect that it will happen if this method becomes popular), the same is true with OCR and images. So, I'm going to stick with the text method for a while and see how it goes.
You can get my updated captcha module here.