The current captchas are very innaccessible, for several reasons:
*The captchas distinguish between uppercase and lowercase. For starters, the user is not even told that the distinction is important! Secondly, such distinction is often very difficult - how is the user supposed to tell the difference between W and w, V and v, etc, when the letter sizes are random?
*The green captcha is inaccessible to many colour-blind people (including myself). 10% of males are colour-blind.
*Visual captchas are inaccessible to users with images turned off (low bandwidth), users with non-visual browsers, and blind or partly-blind users.
At the very least an alternative, audio captcha must be made available. Ideally any visual captcha used should be straightforward and easily usable for any sighted user. Either only the monochromatic captcha should be used, without case distinction, or another more accessible captcha should be used.
Until this bug is fixed, a significant proportion of users will not even be able to register an account. I had to type in my password four times before I got a captcha I could read at all. [originally posted on jforum.net by gracefool]