While I understand the need to validate sign-ups are human, these CAPTCHA systems are horribly discriminatory towards blind or visually impaired users. Of course, to be fair, so are flash-only web sites. They can't read them, and they are blocked from using these sites.

If your web site does not work in lynx/links, it probably doesn't work with a screen reader either -- and it is broken.

You also need to consider the potential for color-blindness in users as well.

I think it was once proposed that you could create riddles involving trivially simple problems (but ones that were not trivially parseable).. but that two has problems, particularly with non-native speakers or those that won't get your riddle. They would probably be annoyed. Seemingly good multiple choice question barrages like "which one of these animals lives in the ocean" seem effective, but you can code around those. And again, what if they don't have flounders in Outer Mongolia?

I realize I am not solving your problem -- I'm only stating that CAPTCHA systems are a very bad idea.