There are some companies around it like * http://www.adaptcha.com/ * http://www.captchaad.com/
But I've never seen it in the wild. The idea seems intriguing. More accessible to my grand parents and a new way to cram more advertising in web pages = make more money.
I haven't seen them in the wild though. Do you know why?
Now since there aren't infinite string variations you are left with randomly displaying only 5 kinds of verification phrases which are easier to manipulate once you take into account that colors associated with brand logos are easier to recognize than actual alphanumeric strings displayed as images.
Also since captcha's are mostly for the benefit of the website/webmaster, they are seen in an annoying light by consumers who actually use the site. And that is a strict no-no to any marketing strategy.
This is more practical (and cuter) than using brand-logos.. Pictures of kittens and other animals are easy to find, whereas getting a wide enough range of companies willing to let you use their logos will be far more difficult.. It would be like having a captcha where there are only 5 possible strings to enter..
I've been asked to work on something similar, but my lazy/distracted ass hasn't gotten around to it yet.