Captcha's do not provide 100% protection against automated bots. Many captcha's, even ones that seem very hard, have been cracked. The Coding Horror blog has a number of articles about this. See for example CAPTCHA is Dead, Long Live CAPTCHA!
Apparently there are smart hackers who know how to write software to recognize the letters in a captcha, even if the letters are (almost) unreadable by a human being.
The Daily WTF also has some funny articles about CAPTCHA failures: