John Battelle spotted a post from Chris Marino at Tumbling Duke that has the worrisome suggestion that Google is allowing third parties to set cookies based on searches people do. But I dropped an IM to Dave Naylor, who immediately spotted this being due to Firefox prefetching.
If you use Firefox, Google will automatically preload the pages showing in the top search results. They made this change back in March. As they warned back then:
With prefetching enabled, you may end up with cookies and web pages in your web browser's cache from web sites that you did not click on since prefetching happens automatically when you view Google search results pages. You can delete these files by clearing your browser's cache and cookies.
So in Chris's case, he writes about how he searched for cars, Amazon and Walmart and got cookies from Cars.com, Amazon.com and Walmart. He assumed this is all related to AdWords in some way.
AdWords isn't the issue. It's because for a search on cars, Cars.com was the first site listed and so that page was preloaded -- and that meant a cookie from Cars.com came with it. It's the same situation was true for Amazon and Walmart. in searches on their names.