Jim Thatcher has written a post on CAPTCHA accessibility. A must read if you're working in the accessibility field.
The whole CAPTCHA issue is a can of worms. They are appearing more frequently by the day, whether to sign up for email accounts or to play games. To avoid spam (they assert) some have added CAPTCHAs to the process of commenting on Blog entries. The letters stand for "Completely Automated Public Turing test to tell Computers and Humans Apart".