i'm not a coder but what are you saying about this to take a module for captcha http://freshmeat.net/p/captchaphp
There is a captcha module based on recaptcha http://www.silverstripe.org/recaptcha-module/
Also, you can add MathSpamProtection::setEnabled(); to your config file which will add a basic math question to your comments form.
I tried with MathSpamProtection::setEnabled(); more then one time, but it doesn't work, i try with version 2.2.2 and 2.2.3.
Before I posted i've seen the recaptch a module, but i want something different ... let me see if I can found something else ....