In my opinion, there are several reasons why [Captcha](http://en.wikipedia.org/wiki/Captcha) is not the way to go for combating blog spam:
* It is an accessibility disaster. It could discourage a disabled person from posting a comment. Worse, it could also discourage a normal person from posting a comment :).
* It can be easily circumvented by using a popular third party web site. For example a porn site. Before a visitor can gain access to the porn site, he needs to give answer to a reused Captcha.
* It does nothing to prevent [Trackback](http://en.wikipedia.org/wiki/TrackBack) and [Pingback](http://en.wikipedia.org/wiki/Pingback) spam.
People, please disable your Captchas and turn to other method of combating spam :).