In reply to RT Cunningham.

A problem I experienced with that approach, is that when people have a cookie issue, they can't comment. It's also not as effective as requiring JS, as a cookie seems to be easier for spammers to fake.

I made a plugin which works around this, by providing a fallback to a CAPTCHA when the user can't run either cookies or JS (it also does some JS trickery to see if the visitor can handle JS calculations) ...