A common method of limiting access to services made available over the Web is visual verification of a bitmapped image. This presents a major problem to users who are blind, have low vision, or have a learning disability such as dyslexia. The document Inaccessibility of Captcha (external) examines a number of potential solutions that allow systems to test for human users while preserving access by users with disabilities.
A description of the problem and some possible sollutions are discussed in the article In Search Of The Perfect CAPTCHA (external).
Many people say that a CAPTCHA is necesary to prevent themselves from spam. The articles Spam-free accessible forms (external) and Captcha-les security (external) should convince them of the contrary.