Due to the mass amount of spam that has been hitting my comments, I decided it was time to put a stop to it. I have already implemented a blog comment box with reCAPTCHA on a different website with cakePHP as a plugin so it was fairly easy to move it over here. The validation is handled automatically when you validate the model and it is customizable so you can attach the behavior to any model you like.
The plugin for cakePHP can be found here. If you are interested in seeing the modifications I made to attach the validation to a model outside of the controllers base model I might take the time to create a branch for my version. Please email me if you are interested!