I don't remember if you do or not, and I haven't read through the thread, but having an audio option for the CAPTCHA would solve this problem.
Also, I've seen some CAPTCHA's that simply ask the user a random question that could only be answered by a human. For example, "What is the day before Tuesday?"
I prefer the latter approach, because you can have such a long list of questions that a spambot would never be able to figure all of them out.
Just my .02 and thanks for the best forum ever, BTW.