The attacks Arduino.cc's has been suffering, are Bot attacks, and bots "cannot" navigate sites protected by "good" implementations of CAPTCHAs, such as the format used over at Yahoo.
It's trivial to break any captcha. You simply link it into a "free porn" site and have users answer the captchas. They type the captcha, the bot automatically enters it into the victim site, and if it's right, the script lets the viewer have their porn. There is no such thing as a good implementation because any captcha can be defeated with this sort of attack.