I'm not at all sure what Google is up to or if the slant of this WSJ article is incorrect.
"Google Inc. and Yahoo Inc. are in talks with the Justice Department in an effort to head off an antitrust challenge to their proposed advertising agreement....
Major advertisers have raised objections to the Google-Yahoo deal, which would align the two largest players in Internet ads. Advertisers have told Justice Department officials that the partnership will limit competition, raise prices and reduce choices."
I'm no antitrust lawyer but it seems to me that the last sentence above is exactly the reason that antitrust laws are on the books. Google has an awesome search engine though, and a revenue stream that is absolutely invisible to 99% of mankind, so they're just going to get on with taking over the world.