Yahoo yesterday announced a deal with Google, that allows them to display Google advertisements in Yahoo search results and other search related products. In order to improve revenues, Yahoo is going to display Google powered advertisements instead of showing their own advertisements.
If antitrust regulators approve this partnership then Google is going display ads in Yahoo, the No 2 search engine. Google is already showing ads in Ask.com and AOL that are in 4th and 5th ranks of search market.
Google is going to get control on 4 out of 5 top search engines which is not a good thing to happen. This shows the monopoly of Google in search and online advertising market.
You can read more about Yahoo & Google deal here