I saw this in use on someones blog today: http://www.webdesignbeach.com/beachbar/ajax-fancy-captcha-jquery-plugin
I've had a long standing hatred of Captcha's, even though they are a necessary evil. It frustrates me when I have to squint to try and read the text, or when I stuff it up 2 or 3 times. It's a barrier to user contribution, but unfortunately due to the nature of the internet we need to use it.
I quite liked the look of this fancy captcha plugin though - simple, stuff you would do as a child, yet should be fine to block the bots. It's a jquery plugin too, I believe that stack overflow uses that.
If this plugin isn't quite right, perhaps a similar concept could be developed by stack overflow?