Another idea along these lines would be to present a scene and ask the user how many objects are in the scene. It's a very difficult problem for computers to descern individual objects in a scene, especially if they overlap.

Do captcha bots ever use the audio string? Could you not hook up a voice recognizer to the audio portion?

That's true. Maybe you could do something with profiles from FB. Show a small collections of faces one of which is your friend on FB. Then you have to select your friend. For the visually impaired you could read peoples' names.

A real person would instantly recognize one of their friends and even the guy getting paid to solve captchas would have a hard time.

Of course, you then have the problem of getting someone to connect FB to the CAPTCHA service for the first time instead of abandoning the form.