"So what about the rest? Two advertising companies took overt steps to respect the Do Not Track headers sent by browsers like Firefox, Internet Explorer, and Safari, which we just learned is actually a step beyond NAI's baseline requirement. Another 10 companies went even further by stopping the tracking and removing the cookies altogether (and just for interest's sake, it's worth noting that Google falls into this category)."
When a consumer visits Google's opt out page (where you obtain a doubleclick.net opt out cookie), or gets one via the NAI, the doubleclick.net tracking ID is deleted.
"So what about the rest? Two advertising companies took overt steps to respect the Do Not Track headers sent by browsers like Firefox, Internet Explorer, and Safari, which we just learned is actually a step beyond NAI's baseline requirement. Another 10 companies went even further by stopping the tracking and removing the cookies altogether (and just for interest's sake, it's worth noting that Google falls into this category)."
http://arstechnica.com/tech-policy/news/2011/07/study-finds-...