Raven on Microsoft's Live Search spotted that it’s indexing Google AdWords ads. The example in the below screenshot shows a search on colorations paint, which returned Google AdWords ad in the 3rd and 4th position.
I checked Google, Yahoo and Ask to see if they were indexing any Google AdWords ads, and they were not - at the time of this post. I then did a deeper check on how many Google AdWords ads Microsoft's Live Search was indexing, and I noticed it was over 4 million!
A site:www.google.com/pagead search at Live Search will show you this for yourself. Here is a screen capture of what I see right now:
Google clearly is blocking search engines from indexing that Google directory in their robots.txt file. However, that doesn't prevent Live from having URL-based listings that come not from crawling the links but merely seeing them from other pages (Google does the same thing -- it may list a blocked URL.
Source:→ SEL Blog