Google has a great post out on its official blog from T.V. Raman. I've heard about T.V. Raman from a fellow SEO/Usability friend of mine, Matt Bailey. Matt was to meet with T.V. Raman at the Google Dance this past year, and he told me a little bit about this man, that little bit had me amazed.
T.V. Raman does a great job at giving a great definition about CAPTCHAs and how the visually impaired can use the audio CAPTCHA Google now provides. He also points out that it's not just for the blind, but great for use when you can't quite make out those funky graphic letters/numbers or when you are on a browser that is strictly text based with no graphics.