Also I personally did stumble to Google's captcha in quite a few sites (borderline porn) posing it as their own for registration purposes (sometimes monochromed though). Google has easily recognizable captcha.
A few years back (probably in 2006 or 2007) I talked to a bunch of researches who were working on image recongition and they told me they searched high and low for any proof of this and didn't find it. So I have had a cached opinion since then until about 5 minutes ago. :-)