The folks at Twitter have now added an audio CAPTCHA to their signup process, thus affording blind and visually impaired people the opportunity to register and participate alongside their sighted peers. See New: Accessible Captcha with Recaptcha for all the exciting details.
I am definitely thrilled to hear that twitter has done this as it is one more company doing the right thing to eliminate unnecessary barriers. It is also helping another great cause at the same time! Audio capchas are fun to do because they represent the elimination of that barrier and in this case make progress towards digitizing the books. I was sitting there solving the audio challenges on the recapcha site for a while just for that reason. Now if every site and company didn’t have to be a fight to get them to implement this stuff the capcha issue would no longer be an problem!