This is a raise of interest, I would mainly like to share this with Daniel as he is away right now but everybody else can comment anyway
While working on some new templates I did some quick Googling on Captcha engines, and managed to find one that is in Joomla-extension form for CB registration and a few other things.
It has a choice of using three opensource/free engines - SecurImage, ReCAPTCHA or (my personal favorite) MathGuard.
I know you are thinking about integrating new Captcha into a later JoomlaComment version Daniel, perhaps it would be easier to integrate this extension?
I was looking at the code myself to see if I could do it, but it's a liitle bit advanced for me
What do you think?
EDIT: The SecurImage is pretty decent for a simple Captcha engine as a new default, and is Lesser GNU v2.1 licence.
is the gallery for that engine, the one at
is actually very good according to my own knowledge of spambots - it uses GDF fonts so the "dots" in the font are random, it really messes with Spambot OCR.