Checkbox, checkbox spambot, and more powerful options for spam control?
Using Jamroom
one of the systems I've heard about for getting around the captcha is to outsource it to people looking for stuff. The spam bot shows the captcha to users looking for other things that dont require a captcha, then once they have filled in the captcha on behalf of the spam bot they are allowed to continue on their path while the spambot has passed the captcha.
The "are you human?" checkbox is added to the page after the page has been generated so it doesnt load for bots so they dont know to fill it out because its not there, so even thought it seams simple there is a lot going on behind the scenes.