Screen readers, in general, will spell out the garbage strings, making it easy enough for the blind to find them and comply. Those who are both blind and deaf can find the garbage string via their Braille terminals... or so goes the theory, since so far no one who's both blind and deaf has contacted me. This was true even before my CAPTCHA went online, though.
It's an easy enough thing to do. From a predefined list of sentences (which could come out of the fortune program), select a sentence at random and a random point within that sentence in which to plug a garbage string. Generate the garbage string, test that it doesn't exist in a dictionary, and plant it in that random spot. Explain to the user that he's supposed to find that non-word and type it into the text input. Use caching similar to that of Authen::Captcha to keep track of what's been recently served and to whom. Bingo bango bongo, an accessible CAPTCHA.
Nuthin' to it but to do it.